Subterranean Thoughts

So I recently took greater notice that many of my favorite gaming writers will post little snippets of what they've been working on to their blog. Even if it's incomplete, it's still a solid preview, can build hype, and is probably somewhat usable on its own. So why don't I try that?

My home campaign setting is called Underworld, and is all about that sweet, dank Underdark, baby. When I first started working on it, I spent a long time thinking about how to bring more out of the Underdark experience in D&D. How much depth would need to be added in order for that one trope to carry an entire campaign? And I like thinking in terms of rules and mechanics at least as much as lore, when it comes to worldbuilding and reinforcing themes.

The following materials were mostly written around 2016 and 2017 but I was heavily sidetracked by worldbuilding and "higher level" game design. Oh, and getting a bachelor's degree. But I consider all of this to be stuff that I one day will return to and do justice, because it's important (in fact, I made a couple edits as I copy/pasted it here). I anticipate eventually either putting this stuff into a Brave supplement that'll have rules, systems, and tables for Underworld stuff, or I might just compile and publish my setting outright and put this stuff there instead. Contained within the following thoughts are some implied setting assumptions that may not be true for the "default, vanilla" Underdark, but you'll still enjoy it. Plus, I included lots of great pictures you'll want to expand.

And yes, the release of Veins of the Earth did stifle me a bit. But while that book is indeed excellent, I also think you'll agree that much of what follows builds onto it quite nicely and covers things it doesn't address.

The Three Main Principles of Subterranea

The most important thing to nail when running a game in the Underworld is to consider how the differences in the space you're occupying change the way you experience the game. When you’re underground, the moment-to-moment act of navigating space can be very different than what you’re used to. I have identified three principles that distinguish this environment from the Overworld in ways that'll strongly shape your understanding of the world around you and the decisions you make within it.

1. Amplification
2. Zone Limit
3. Verticality

Let's talk about these one by one.

Everything you already know needs to be translated through Amplification in the Underworld. Simply put, this place is more extreme than the Overworld. Small becomes tiny and big becomes huge. Rough becomes savage and refined becomes angelic.

Zone Limit is a term I needed to make up in order to describe a certain quality that's a bit unintuitive for Overworlders. You need to reverse your understanding of space and the stuff occupying it. When you live on the Overworld, you enjoy this vast, wide, open sky nearly everywhere you go, and all the "stuff" is either right below you or in flimsy structures to your sides. But in the Underworld, space is occupid by solid material by default. Hollow spaces are an exception to that. It's really cramped down here. Thus, we say that the Underworld is characterized by a high "Zone Limit," and the varying degrees of Zone Limitation have a huge impact.

Lastly, dwellers of the dark must consider Verticality at all times. This is probably the most obvious one, but is also notoriously difficult to gamify easily. You might treat it as a smooth spectrum, with the Z-Axis as equally navigable as the X and Y Axes, but I would generally recommend that you stratify the Z-Axis into discrete levels for simplicity.

What are the implications of these three principles? Well, let's review some examples of how each one applies:

Amplification:

1. Cover in combat has greater consequences. On the Overworld, you might get a wall that keeps someone from targeting you directly, but they can usually either get around it or knock it down. In the Underworld, whatever source of cover you have will probably be covering most of you (because of Zone Limit), and oftentimes has the full structural integrity of the Earth itself backing it up. But if they do manage to break it down? Almost-guaranteed death.
2. The choice to use light has greater consequences. On the Overworld, you need it to see in the darkness but sometimes worry that hostile NPCs might notice your light if you have it out in the open. In the Underworld, many monsters are blind and you don't have to worry about using light at all. Your concerns are instead with being quiet and covering your scent. Even better, if something isn't quite blind, but has sunlight sensitivity, then your campfire might actually ward them off! Ah, yes, but if there are hostile NPCs or monsters with darkvision, like many enemy forces, then your light will probably illuminate the whole cave (since now you have a ceiling to worry about). If you want an army to move stealthily, then they may need to rely instead on communication by touch or whisper (for which entire languages have been created).
3. Resource management has greater consequences. This place is weird and absurdly dangerous, and much of the landscape is barren. But things that do thrive have more life and power than anything found on the surface. Subsistence agriculture yields much greater results than your puny Overworld crops, but it’s a rare opportunity. Raiding the surface world to get supplies might be easier than farming for many societies. It would especially become more common for a community ravaged by war (which is common). And of course that means taking surface-worlders as slaves or conscripting them into your army/mercenary company.
4. Dealing with monsters is much more common and terrifying. This is their home and they are everywhere and you can barely understand them. They grow bigger here, they hide easily here, they have sensory perception that you don't have, and so on. Armies are trained at least as much for fighting monsters as they are for fighting other armies.
5. The temperature is much colder by default than on most of the surface world because of the lack of sunlight. Don't get me wrong, there's plenty of sources of geothermal heat. But they mostly exist in little pockets scattered throughout. 
6. Fires are much riskier because there's already a limited oxygen supply down here. And consider the kinds combustible organic materials you use to fuel fire, like wood. Down where everything is stone, that's much more valuable.

Zone Limit:

1. Long-range archery isn’t common. Not enough room.
2. Open-field warfare in general isn’t common. Again, not enough room. Hit-and-run tactics are more common, as is trench warfare but with tunnels instead.
3. It's easier to conceal things underground, whether they be locations or people or whatever. Small pockets in the opaque ocean of the earth are inconspicuous. Looking for something is always a "needle in a haystack" situation.
4. Once a tunnel is occupied you can’t really go around or spread out, and you can’t fire ahead past the other occupants easily. Instead, you just have to push through a tunnel piece by piece until you reach the end. “Tunnel Push” is a fundamental of combat and figures into most battlefield tactics.
5. Having the power to reduce and/or increase Zone Limit is a game changer. The vast majority of the time, you're looking for ways to reduce Zone Limit and increase your options for mobility. Soft reductions might be building bridges and ladders, but hard reductions may entail using explosives and digging equipment to just create new tunnels entirely. However, occasionally you have reason to increase Zone Limit, usually to inconvenience an opponent. Building walls would be a soft increase, but using explosives to collapse tunnels would be a hard increase.
6. However, there might be regions with extremely high Zone Limit where it’s illegal to increase it further. Using bombs in these areas could be prosecuted as a crime under a sort of Underworld common law. Libertarians may have trouble understanding why we need a common, shared road system, but denizens of the Underworld understand perfectly.
7. Large-scale strategy has to account for large-scale Zone Limit. When you reach the parts of the Underworld where things are more isolated, especially near the Plates, there may be only one tunnel to and from a city, or only one tunnel that connects two cities (without having to go around for months). Conversely, a city with low Zone Limit has lots of advantages and vulnerabilities that come along, too.

Verticality:

1. Verticality is a double-edged sword. It's nice because it functions as an (often unintuitive) counter to Zone Limit. You can plan your attacks going upward or downward now instead of only ever having lateral options of approach. On the flipside, you have to be prepared to defend yourself from above and below, too. Players should get into the regular habit of always asking, "what do I see above us?" and "what do I see below us?"
2. Fortifications must be built with verticality in mind, since you can target the structural integrity of a building by digging out its foundation from below. Societies with more advanced engineering may have architecture that is spherical and uniform in its support.
3. Verticality opens up options in combat. Giants and trolls and other big bois are nearly unstoppable on the Overworld, but in the Underworld you simply have to rely on the Z-Axis more. Just carve out a foxhole in the ceiling/walls, above head level for the giant, and be ready to jump out on top of them when they pass underneath. With an iron stake and hammer, pound it right into their brain through the skull for (hopefully), an instant kill. Lace it with acid I guess if it’s a troll. Consider options like that as often as possible.
4. Verticality means you'll be doing lots of climbing. There won't always be stairs and elevators. Have the fall damage rules handy at all times, and get used to running combat from a side-view rather than a top-down view. What do I mean by that? Okay, let's look at a couple pictures.

Your standard combat setup, right? The grid represents the ground, and each person is standing upright and moving laterally. But now check this out:

Combat while climbing flips things on its side. Now, the grid represents the clifface, and the minis are flipped over. Here I drew some narrow walkways winding up the cliff, but you could also put in some handholds and vines to climb and whatnot. The isopod is "upright" because it's climbing on the side of the cliff, the troglodyte probably popped out of one of those holes in the wall, and the harpy is hovering 5 or 10 feet away from the wall. Try running some combats like this, it's cool.

But my favorite thing about the three principles is all the places where they crossover:

1. 
   The pike (and other weapons with reach) makes for great Tunnel Push, because its benefits are Amplified in the tunnels. However, you’ll need a double-ended weapon to account for the limited space in being able to swing it around, in case enemies come from behind (Zone Limit). Many Overworlders have been killed because they weren’t equipped for the environment and their polearm was pointed in the wrong direction.
• I kind of love that I found a reasonable in-universe justification for the inclusion of double-ended weapons, one of my least favorite fantasy tropes that I'm glad didn't survive past 3rd Edition.
2. Cavalry is similarly subject to Amplification and Zone Limit. You rarely get enough width of space for a cavalry line, but an individual charging down a tunnel can make just as huge of an impact. Nowhere to run from its hooves.
3. The ramifications of dividing your assets is Amplified. Zone Limit means that once your infantry and cavalry have been split up, they can’t rendezvous until the caves meet again.
4. The importance of scouting, surveying, and cartography is vastly Amplified because of the effects of Zone Limit and Verticality. Different societies have innovated different methods for mapping the 3-Dimensional tunnels. Access to this is vital in navigating the Underworld without going mad. Correspondingly, disrupting an opponent’s means of surveying gives a huge advantage. Don't get me wrong, GEOINT is pretty dang important in Overworld warfare. But we often take for granted that we can accomplish even the most basic of functions in warfare (e.g. moving from point A to point B, setting up camp, communicating messages, etc.) only because we enjoy similarly basic knowledge of what stuff exists around us.
5. The danger of flooding is Amplified because of Zone Limit. If a whole tunnel floods, then there's no surface you can even swim to for air.
6. Flanking an opponent is Amplified because of Zone Limit. Forcing your enemy into a two-way Tunnel Push makes a proper rout or withdrawal impossible. Underworld combat is far more likely to result in a bloodbath than we're used to.
7. 
   Trapping an opponent is Amplified because of Zone Limit. Leading your enemy into a chamber and then collapsing it or opening a poison gas pocket to be released in is a huge danger.
8. This can be built on further with Verticality. You can collapse a floor or pocket onto a tunnel below, or set a fire to smoke out a tunnel above you. Be careful about increasing Zone Limit or accidentally lighting deep earth gases, though. Also consider redirecting the flow of lava tubes or draining a lake and flooding caverns and cities below.
9. Accounting for monster territory gets Amplified because of Zone Limit. Monsters affected by war might be displaced from their normal habitats, often enough to see a monster far, far outside its usual homeland. Unstable ecosystems mean that scavengers are much more common. You can lure a monster towards an enemy with bait, or keep it away with wards or chemicals.
10. Verticality is Amplified when it comes to the relationship between the Overworld and the Underworld. The Overworld serves as a Zone-Limit-free level where transportation can be quick and safe, but foreign, conspicuous, and usually distant. It is often treated like "air support" for some of the higher Underworld armies. For example, some dwarves fighting some hobgoblins will still need to focus most of their efforts downwards, since the enemy is below them. But they might station a small number of units around the Overworld anyway, just because they can move around much more quickly and freely, and thus, can transport messages and supplies to other near-surface units more readily.
    

As you can probably tell, so far I've really only focused on combat and warfare. But you could probably think of lots of things these principles apply to.

Topography

Pivoting over to a different subject entirely, one of the first things I thought of for "how do you flesh out the Underdark" was to create different "terrain types" for it. See, in most versions of D&D where the Ranger class or Druid class gets some kind of "favored terrain" feature, one of the options is almost always "Underdark" or "dungeon" or something. And this carries two problems. 1) It makes this environment seem awfully uniform, even though it should be vastly greater in scale than the entire Overworld (which is quite literally like the tip of an iceberg poking out above the waterline), and 2) a campaign set largely within the Underworld would make such a choice a bit overpowered. I also remember reading the 4E D&D Underdark supplement book as a wee lad and recall this being one of its cooler features (although revisiting it now, it's definitely got a lot less than I remember). Thus, let's add some variety to our Underworld instead of only ever using limestone caves.

One of the stranger creative choices I've made here is to blend "terrain" with... well, something more like "planes" or "dimensions." See, I want my setting to be very mythology-inspired. It's not just some Jules Verne-style caving adventures. This should be the Underworld of Ancient Greece or the Bible, y'know? So the word I use for this category is "Realm." There are 9 types of Realm in the Underworld, each with 3 subtypes. Some are more mundane and some are more magical and otherworldly.

1. 
   Cavern: the most generic and common Realm. It’s what you think of normally when you imagine a subterranean environment. Stalactites, stalagmites, bats, glowing mushrooms, echoes, flowstone, etc. Wide, open spaces aren’t common. Weather effects include mass migrating luminescent lichens (light+food), sludge/mud slides, and maybe some spore clouds.
1. Warren: limestone, marble, or dolomite caves healthy in mineral deposits, and thus, sapient life. Most of the stuff you'll find out there for Underdark stuff can be applied here.
2. 
   Sand ward: sandy tunnels that collapse easily. Water is rare, air is still, sound is muffled, burrowing monsters are common. It's easy to hide traps, but it's also easy to slip and get buried in a dune. Don't forget quicksand.
3. Crystal bed: enormous gypsum columns the size of a tree. They’re varying degrees of translucent and reflective, so they also Amplify light and sound.
2. Magma Chamber: the one where you have to deal with magma and overheating. Found inside volcanoes and mountains, below the seafloor, in various places around the Abyss, and anywhere else you want to throw in a pocket of heat in the earth. Some typical weather events include lava flooding, geothermal waves, and steam storms.
1. 
   Steam channel: fields of steam geysers that are like dangerous saunas of geothermal vapor. This is also a good source of water in the Underworld, even if it’s boiling.
2. Volcanic rift: fissure vents created by shifting magma, resulting in a lot of exposed lava surfaces and maybe even a growing lava pool. Expect seismic activity and sudden exploding lava pockets in the floor, walls, and ceiling. 
3. 
   Fire vortex: when a source of fuel catches fire, such as a seam of coal or the corpse of a fatty beast like a deep-sea whale, it can last for years, decades, even centuries. A miles-long shaft underground filled with a column of fire, or even the smoke-filled chamber you have to pass through that sits just above a cavity of raging fire, can be nearly impossible to navigate without the right equipment.
3. Ice Arena: the necessary "ice level" of any good game, cold environments can form underground in a greater variety of conditions than you might expect. In fact, the general trend as you go downwards will be for the Caverns to gradually give way to Ice Arenas and Magma Chambers, with other Realm types breaking it up.
1. 
   Frost cave: cold caves, permafrost on the ground, big ice formations. These usually form if they're being fed by a water source somehow. Occasionally there'll be pockets of extreme, uninhabitable cold because of the evaporation cooling effect that can happen at the boundary between a moist frost cave and a warmer cave with dry air (a "cold trap" I believe it's called).
2. Glacier hollow: literally the tunnels that form in a big hunk of ice, like a Swiss cheese ice cube. Just walking around is a struggle, and a Magma Chamber may be closer than you think. Often unstable due to melting and glacial motion (i.e. "icequakes").
3. Blizzard zone: this is that Jules Verne shit. Occasionally there'll be massive enough chambers underground for precipitation to form, and terrible cold storms are common. Really long-lasting storms in really open areas can form seas of methane that, like Venus, can host floating structures at the right altitude.
4. 
   Dungeon: does this count? I put it on the list because I imagine dungeons that span literally miles of area being a feature of the Underworld. What defines this Realm type from the others is that it was shaped by sapient creatures in some way, and still follows their logic and purposes. And there tend to be lots of leftover traps and treasure.
1. Ruin: abandoned areas of frequent activity. Settlements, mines, fortifications, prisons, tombs, vaults, archives, etc. These usually have a lot of history to them and, especially, leftover lore. They're also the most-frequently claimed by sapient creatures who want to repurpose them as their own territory
2. Maze: as longtime readers of this blog know, I kinda have a thing for labyrinths. A single maze dungeon is cool. A maze expansive enough to fill out a 24-mile hex is crazy. And to think that there's more than one of those in the world? I would imagine most of these mega-mazes could be attributed to the efforts of duergar and probably often serve as prisons, patrolled by lots of constructs.
3. Sewer: the main thing setting sewers apart from other dungeon types is that they're the aquatic variety. There's a big channel of water running down the middle of every hall, and there are drainage pipes everywhere you look.
5. 
   Waterbody: these present many of the same gameplay considerations underground as they do above ground. Characters are mostly limited to moving around on vehicles, there's a lot less opportunity for combat or exploring stuff (except for the islands dotting the waterbody), and you'll need to keep the drowning rules on standby. The main difference is that you're a lot more likely to find yourself adventuring under the water when in the Underworld. Ever heard of Thalassophobia? I believe the deep sea should be considered a part of the Underworld equal to all the caves in the ground, and thus, we should have plenty of adventures dealing with fish monsters and fish people. Raining and flooding are still concerns down here, as are waterspouts and freezing.
   
1. Darksea: you better make those under-sea kingdoms fucking enchanted, you hear me? But also, really fucked up. Invest in some minis of deep sea creatures. I got this pack at Michaels that has a hatchet fish, angler fish, viper fish, glass squid, giant isopod, gulper eel and black dragonfish. And don't be afraid to give your Underdark societies some submarines. I know it sounds pretty sci-fi but I think it's inevitable. Pick out your favorite "space combat" rules from a sci-fi game and repurpose them for under the waves.
2. 
   River: the main attraction here of course is the Styx, which is a colossal river that winds around every corner of the world, gradually forming a downard spiral that lets out in the center of the earth. That said, in order to have an excuse to include the Styx no matter where in the world my party is adventuring, I tend to characterize it less like the Nile and more like the Amazon, where it's a huge system of hundreds or thousands of tributaries. Which means I'll probably include the other traditional mythic rivers as major tributaries: Acheron, Cocytus, Lethe, Phlegethon.
3. Reef: coral, oyster, sponge, and so on can all substitute for stone in the structures and caverns that form under water. Being the most abundant collection of biomass you're likely to find down here, there's always some farmers carefully tending the fields of reef. This also means that reefs tend to be well-fortified and defended by garrisons. 
6. 
   Salt Desert: there was once a layer of primordial oceans over the world, but then the gods built on top of it. Most of those oceans slowly evaporated and their water never precipitated back down, instead rising higher in the Underworld. What remains are vast stretches of dry salt pans. Because of their openness, these Salt Deserts accomodate greater cavalry armies and battles than any other Realm type, meaning that leftover battlefields are also quite common. Of course, the transition from salt pan to other Realm types still usually involves long tunnels of salt, where the beautiful range of colors is more apparent.
1. 
   Erg: it's pretty much like a sand desert on the Overworld but even worse for your hydration. Complete with salt storms, mirages, and dunes.
2. Salt dome: like the salt domes we have on Earth, but way bigger. Amplification and all that. These are basically "salt mountain ranges" that break up the deserts, although more prone to shifting and growth. These also show off the colorful strata of salt.
3. Halosteppe: tending to form more from dried lakes than oceans, some grass-like life has managed to adapt to this environment. This grass can conceal mud quagmires that swallow up an elephant as well as brine springs.
7. 
   Mist Lands: any range of Underworld Realms that follow your standard "fairy dreamscape" patterns and are inhabited by most fey. Of course, the Mist Lands are more like "pocket dimensions" that don't strictly fit into the world around them according to normal physics. Hence, why you usually see their borders obscured by mist. Throughout the Mist Lands you'll run into strange weather patterns like mirages, time fluctuations, and sentient flower blooms.
1. 
   Mushroom forest: I mean, you gotta have fungi forests, right? Giant mushrooms and all that? Get a good list of fantasy mushrooms to color your world. The ones from Forgotten Realms aren't bad, although at some point I'm sure I'll make my own.
2. Iron swamp: to me, swamps are the ultimate hostile environment. Combine that with the Underworld, and no sensible adventurer would ever dare to venture into an iron swamp. Of course, do they still have trees? Do I just use mushrooms again? No, I'm actually picturing the major flora of the iron swamp to be rods of metal ore standing upright in tree-like formations. Take your normal bog iron and extrapolate a whole
   
   biome out of it, with rust-rivers flowing between the metal reeds and the occasional spark lighting up dangerous gases. Be prepared to fight a banshee.
3. Wonderland: almost like "civilization," at least from the Fey's point of view. The grass grows in checkerboard patterns and MC Escher stairs surround you and common objects stand in for other common objects in weird, dreamlike ways. I tend to flesh out these areas like fancy walking gardens that extend for miles.
8. 
   Netherworld: the evil version of the Mist Lands, duh. I never liked the Shadowfell much, so I'm remixing the undead plane with the "dark fey" stuff and some other ideas. The Netherworld is an ugly, dark reflection of all the parts of our imaginations that get celebrated in the Mist Lands. Weather might include nightmare apparitions, whispered blasphemies on the wind, a blight degrading your health and fresh supplies, that kind of thing. 
1. Vale of Shadow: spooky, otherworldly dark forest that you probably unknowingly entered just by getting lost in the regular woods on the Overworld. I try to draw from Twin Peaks to get it
   
   right. And yes, in this case I'll use trees instead of mushrooms because I decided that the corpses of dark elves naturally sprout into evil-looking Snow White trees. Which, yes, implies that every Vale of Shadow is actually an overgrown battlefield where many dark elves lay dead.
2. Boneyard: like Dark Souls at best and a Zdzisław Beksiński painting at worst. Filled with undead, both corporeal and incorporeal, as well as some dark gods of rest. For weather effects, some days there's just pure silence, or sadness, or recurring memories.
3. Nightmare: If the Vale of Shadow is the woods from Twin Peaks then Nightmare is the Black Lodge. Or, like, a Francis Bacon painting. Temperature is unnaturally cold and makes you feel like you have a fever. The walls and passages are cast in stark light and shadow without explanation for the
   
   borders. You get a growing sense of despair and unease if you spend too long there. All nonmagical drinks might turn into wine. If you stare into your reflection in the water, you feel compelled to fall in. When you reappear, you'll be temporarily deafened for some reason.
9. Hellscape: yes, you will indeed step right into Hell itself (or perhaps, a hell) if you just walk down far enough. Expect lots of demons and tortured souls. I think you should feel free to mix-and-match any sort of hells you want and throw all of it in. All 9 Hells of Baator, every infinite layer of the Abyss, all 12,800 Chinese hells, the hell from Adventure Time, whatever. Some of the typical "weather" you deal with is elemental storms, blood rain/flood, and shrieking battle cries everywhere.
1. 

   Elemental chaos: this varies by element type, although mashing them together is the funnest way. This covers your fire and brimstone hell, but also your frozen hell, your windy hell, your mudslide hell, and so on. Pick up Dante's Inferno sometime.
2. War zone: this is the Hieronymus Bosch hell. Maybe my favorite one, the local terrain is literally just armies of demons fighting each other. At the same time, it's not unlike a carnival or a casino, since war is all fun and games for demons.
3. Flesh jungle: this is the gross, body-horror hell where the tunnels are made of skin and teeth. There are piles of reaching bodies stretching out from the floor and all that.
   

I'll admit, I've never been super happy with this list. The first one I'd probably cut is the Dungeon Realm. At the same time, I'm always looking for good additions to expand the list. The standards are pretty high though. The proposed Realm has to be something potent and varied enough to not only have 3 sub-types, but which could fill out a 24-mile hex nicely. A pocket of weird magnetic flux? Pretty cool for a single room or maybe a whole dungeon. Good enough to be a whole Realm type? I don't think so.

For each one, my intention is that they'd get a full page or two with an array of useful, gameable info specific to that Realm type. It might include smells and sounds, a random encounter table, terrain hazards, landforms, associated spells (for Circle of the Land druids), and natural resources that can be used as part of my crafting system. Coming up with such lists is a bit hard though.

Of course, I already included some mention of what kind of weather you'd get in each Realm type. More generally, the table I drafted up for weather effects found throughout the whole Underworld mostly focuses on air quality. Thin air, still air, humid air, rancid air, etc. That said, for those special, juicy "rare weather events," we have things like:

1. 
   Tremors
2. Pitch black
3. Dust clouds
4. Acidic fog
5. Rain/Flood
6. Earthquake
7. Windstorm
8. Earthquake
9. Bug swarms
10. Wild magic surges
11. Ghosts
12. Magnetic flux
13. Electrical storm (in a big enough chamber, the constant cycle of static exchanges can provide bright light for days)

And of course, it's not like I didn't brainstorm a preliminary list of general Underworld features that can be used to inspire a dungeon room or spice up a random encounter. Here are some I wrote down:

1. 
   Sinkhole
2. Lodestone
3. Thermal vent
4. Nuclear reactor
5. Glacier
6. Meteorite
7. Coal pit
8. Ore vein
9. Canyon
10. Chasm
11. Fault line
12. Echo vault
13. Waterfall
14. Trade road
15. Burrow
16. Wellspring
17. Lava tube
18. Pressure ridge
19. Water-filled chamber
20. 
    Muspelheim Elevator (these are giant, miles-long brass elevators that all lead to the fire giant kingdom, which they built so they could more easily monitor their vassal states)
21. Cryovolcano spewing freezing methane or ammonia
22. Bat colony
23. Time-as-space shaft where down=past and up=future
24. Spider web fields
25. Gypsum flowers
26. Moonmilk
27. Geyser
28. Poison gas deposit
29. Fossils
30. Portal (whirlpool?)
31. Convection vents
32. Slime on walls, ceiling, floor (might come alive)
33. Mud pits

So, y'know, really Grade A stuff right there.

Equipment

Even before I got into Knave, I always wanted to make my D&D more equipment-focused. I still feel like a new "Underworld equipment list" could be one of the more valuable things I might offer up. Here's the stuff I brainstormed already:

1. 
   Double-ended weapons (because of the aforementioned Zone Limit problem)
2. Hazard suit (for going through fire vortexes and stuff)
3. Similarly, some kind of super tent, like a nonmagical version of Leomund's Tiny Hut. After all, if you're traveling through a Salt Desert or Hellscape or something and there's a storm of some kind when you wake up, you might just decide it's safer to spend the day hunkered down in safety.
4. Warding and luring chemicals for monsters 
5. Bombs and dynamite and stuff like that
6. Lantern shield
7. Star metal tools and weapons (I later expanded on this idea with many materials)
8. Various magical light sources. I was still thinking in 5E terms, and I came up with the idea for a forehead bindi that can generate light but uses your concentration. I still think a magic bindi would make for a great item, maybe for monks.
9. Perfumes to disguise your presence, since many creatures rely on scent instead of sight. There would probably be one for each Realm type.
10. Different means of surveying. The obvious ones would be sonar and temorsense, but I haven't assigned those to any specific Underworld cultures yet. I have, however come up with these three:
1. Gnomes have a magic food called a Käse Map, which looks like a big hunk of perforated cheese. They set out a bowl of milk, mix in magic acid, and toss in a few local rocks and some dirt. By morning it will have cultivated into a block of swiss cheese that correlates perfectly to the local cave system. The best part is, at the end of the day when you are done with it, you can eat it as a ration.
2. Dark elves keep a bag of live Spyders and let them work overnight. They go out and collect information, then they come back and use their webs to re-recreate the local network of caverns, with each strand corresponding to a tunnel. It’ll take 1 to 8 hours depending on how complicated the system is. They will be noticed by Drow. Need to be fed with maggots but otherwise last forever. If half of them die then the supply is useless.
3. My mutant piggy orcs will always breed a healthy Cartography Hog for each horde to use as a mapping station. There will be designated death scouts connected to the pig by their intestines, pulled out and lodged into the hog. The death scouts then charge as deeply into the tunnels as they can, eating bits of dirt and rock and whatnot along the way, sending back the information through their intestinal tract into the hog, which then dictates an illustrated form of the system to make maps. The hog can also just be consulted on the area and will have a perfect memory of it. This system can become confused, though, if a death scout is killed and someone feeds something weird into the tract. Rocks from a far-off land would start making a map of another place, and I have no idea what other stuff would do.

In addition, I have repurposed Electrum as the primary currency of the Underworld, modifying the rules per this post here.

Conclusion

Trust me, Veins of the Earth isn't the be-all, end-all resource for all things Underdark. I will strive to create more and more content for my favorite D&D setting. Suggestions welcome for ways to expand on the content I've already presented here. I have lots more detail about other setting elements though. Laws, religion, languages, economics, and so on. This is just the "core" Underworld stuff (so far).

Alright alright, just one more picture of Nightmare for you.

-Dwiz