The Underdark

What it is. The Underdark—also called Deepearth, the Lightless Lands, the Night Below—is not one cavern but a continent-sized web of vaults, tunnels, lakes, chasms, and buried seas honeycombing the crust of Toril. It has no capital, no single people, and no central law—only city-states, predatory empires, and old hungers. Echoes exist on parallel planes: the Feydark in the Feywild and the Shadowdark in the Shadowfell.

The three depths (and the “World Above”)

World Above is what Underdarkers call the surface. Its shafts, sinkholes, and ruins are the usual entry points.
Upperdark (0–3 mi / ~0–5 km) is the friction zone: raiders and traders pass both ways; surface folk mine here; monsters hunt by smell and sound.
Middledark (3–10 mi / ~5–16 km) holds most cities and trade routes: drow metropolises, duergar foundries, illithid enclaves, svirfneblin burrows, and kuo-toa grotto-towns.
Lowerdark (10 mi+ / 16 km+) is myth made stone: aboleth seas, elder ruins, lich-vaults, god-scar caverns, and places even the drow whisper about.

“Absence of light results in darkness. So, what results from the absence of Good? The Underdark is a place with very little of either.” —Mordenkainen

Not one world, but many: the domains

You can’t walk from one end of the Underdark to the other. It’s a patchwork of domains—immense cave networks with sparse, perilous links between them, much like continents with few straits.

Northdark (beneath the North and the Sword Coast): Menzoberranzan, Blingdenstone, Gracklstugh, the Darklake.
Earthroot (under the Heartlands): sprawling dwarven delves and drow frontiers.
Deep Wastes: lawless hollows, monster realms, and forgotten roads; the closest thing to a wilderness.
Great Bhaerynden / Old Shanatar: elder dwarven kingdoms turned labyrinthine ossuaries of glory and grief.
Glimmersea: a lightless inland sea ringed by cities and horrors in equal measure.
Darklands, Buried Realms: umbrella terms for farther, deeper reaches where maps go to die.

Across the seas. The Underdark once ran beneath the Trackless Sea, linking Faerûn to Maztica. A cataclysm called Rockfire—drow-wrought Darkfyre that melted stone and flooded miles of tunnels—shattered that corridor, poisoned vast reaches, and left isolated dwarves to become desert dwarves in shallow cave-holds. In the Spellplague’s wake, Laerakond arrived with a sliver of Abeiran Underdark beneath it—different geology, drow, and unknown monsters.

Living stone: topography that refuses to stay put

Expect everything from belly-crawl fissures and cathedral vaults to coral-like stone grown in mineral seas, ice caverns hollowed by thermal veins, fungal “forests,” bone hives, and bubbles of force pinning voids in place. The Underdark changes—lakes drain through quake-cracks; magma opens new roads; rooffalls erase cities. Maps are lies with aspirations.

Environment & climate (why most expeditions fail)

Light & silence. True blackness broken only by ghostly fungus or glowstone. Sound carries oddly; then comes the tomb-quiet that frays nerves.
Air. Stale pockets, CO₂ pits, explosive gases, volcanic fumes. Siphon-tunnels can trap deadly mixes for years.
Water. Abundant but perilous: ice-cold (hypothermia risk), mineral-tainted, or home to things with too many teeth.
Temperature. Stable and cool to clammy in most regions; pockets of blistering heat near magma, bitter chill near ancient ice.
Logistics. Food is scarce off-route. Forays without secured supply points rely on fungus, rothé, and what you can trade or take.

Who lives here (and how they rule)

Peoples (partial): aboleths, beholders, cloakers, drow, duergar, dwarves, illithids (mind flayers), kuo-toa, svirfneblin, rock gnomes, derro, quaggoths, grimlocks, oozes, and stranger things.

Politics: no kingdoms spanning continents—only city-states (autocracies, magocracies, matriarchies, theocracies) connected by uneasy trade. Slavery is grimly common. Alliances are brittle and enforcement ends at the city gate.

Drow rule by noble Houses and priesthoods (Lolth foremost). Cities like Menzoberranzan project power through raids, trade monopolies, and proxy wars.
Duergar build martial, grim foundries: slave-driven industry, rigid hierarchies, and ironclad oaths.
Mind flayers farm thralls for brains and push psionic empires by stealth, glyph, and elder brains.
Svirfneblin and rock gnomes survive by craft, illusion, and the art of not being noticed.
Kuo-toa devolve dogma into reality: what they believe with enough collective madness can sometimes become true.

Trade: Armor, weapons, worked stone, exotic fungi and venoms, relics, magic, slaves. Imports include food, timber, and surface ores. Smugglers and caravaners (often quaggoth-guarded or umber hulk-led) stitch the economy together.

The faerzress factor

A pervasive, scintillant magical radiation—faerzress—soaks many deeps. It nourishes unique fungi and magivores (coral-like base of the food chain), scrambles teleportation and long-range divinations, and causes wild surges and spell quirks. Many cities are sited in faerzress-stable pockets; many disasters begin where it mutates.

Ecology: what eats what when there’s no sun

Primary “producers.” Fungal mats, cave lichens, chemotrophs, and magivores that absorb faerzress.
Grazers & herd-beasts. Rothé (dire musk oxen), blind cave fish, cave lizards, deep roaches.
Predators. Hook horrors, oozes, ropers, cloakers, choker swarms—then up the chain to umber hulks, basilisk packs, and shadow dragons.
Top threats. Aboleth collectives, beholder hives, illithid colonies, driders in Lolth’s wastes, and the whims of demon and devil incursions.

Travel & survival guide (how not to die immediately)

Light is a beacon. Use hooded lamps, glowmoss, or infravision mates; never silhouette yourself in a tunnel.
Air test often. Candles, canaries, or better—alchemical sniffers; crawl low in bad pockets.
Mark routes cleverly. Chalk invites ambush. Use scent, knotted cord codes, or notch systems only your crew understands.
Expect map drift. Plan redundancies, measure slopes, and trust watercourses more than walls.
Mind faerzress. Divination and teleport aren’t reliable; carry mundane contingencies.
Buy safe conduct. Tolls and tokens from local powers beat heroics. An offended House turns a city into a maze you won’t leave.
Eat local carefully. Many fungi are edible; more are soporific, explosive, or alive enough to object.
Silence is a weapon. Sound discipline matters; so does masking scent. Quaggoths have noses.
Carry a story. Down here, reputation is currency: who vouches for you, who you’ve paid, who’ll take offense.

Notable places & patterns

Darklake / Glimmersea: inland black oceans plied by fungal-sailed skiffs and aboleth-haunted barges; cities cling to shore like barnacles.
Beholder hives: caverns sculpted into perfect spheres and rays; trespass and you’ll meet excellent geometry at lethal speeds.
Elder dwarfholds (Old Shanatar): miles of echoing halls, forgotten forges, and oaths still waiting to be answered.
Mind flayer domains: quiet, clean, terrifyingly orderly; everything slants toward the elder brain’s pool.
Fungal forests: pillar-stalks, shelf-trees, and cap-canopies forming whole ecosystems—beautiful, breathable, and occasionally carnivorous.

Cross-plane reflections

Feydark mirrors much of the terrain with grander, crueler whimsy: gem-lit megacaverns, tyrant fomorian kingdoms (Mag Tureah, Harrowhame), and roads that rearrange behind you.
Shadowdark is the Underdark as entropy: colder, meaner, and threaded with undead, shadow oozes, and despair-tuned hazards.