Sulphur cave

A cave sealed off from the world above, where life does without the sun altogether: bacteria feed on the sulphur in warm water and the gas in the air, and grow into floating mats like wet paper. Everything else eats those mats: blind spiders, pale centipedes, water scorpions, leeches and white snails, kinds found nowhere else. The air is warm, short of breath and stinks of rotten eggs, and the water is too foul to drink. Such a cave is real: Movile Cave, in Romania, has been cut off for millions of years and holds dozens of species that live nowhere else.

Light
Dark
Going
Hard

Going down

How warm the rock is and whether this place can grow there, under a temperate land: the deeper, the hotter.

Near the surface (50 m)
11 °C, it grows
A few hundred metres down (400 m)
21 °C, it grows
A kilometre down (1 km)
38 °C, it grows

Hear it

Hazards

  • Foul air. The air holds too little breath and too much gas. Hearts pound after ten steps and a flame burns low and blue. D&D 5e (House rule): Our rule: each hour here, a creature that breathes makes a DC 12 Constitution save or gains 1 Exhaustion level; in the air bells over the water the DC is 15. The Exhaustion gained this way goes after an hour in clean air. Open flames go out after 10 minutes.
  • The warm sump. The deepest part of the cave is reached only by swimming under the rock, through warm water that stinks. D&D 5e: A creature can hold its breath for 1 plus its Constitution modifier minutes (at least 30 seconds). After that it gains 1 Exhaustion level at the end of each of its turns, and loses them all once it can breathe again.
  • Sulphur water. Clear, warm and tempting after the climb down, and it tastes of rotten eggs. D&D 5e (House rule): Our rule: a creature that drinks the cave’s water makes a DC 13 Constitution save or has the Poisoned condition for 1d4 hours. Boiling does not help; magic such as Purify Food and Drink makes it safe.
  • Darkness. No light has reached this cave in an age. Everything that lives here is blind, and does not need to see the party to find it. D&D 5e: Beyond the party’s lights the cave is in Darkness and Heavily Obscured: a creature trying to see into it has the Blinded condition. Darkvision turns Darkness into Dim Light within its range, where Wisdom (Perception) checks that rely on sight have Disadvantage.
  • Slick mats. The floor and the edge of the water are coated with slimy mats of bacteria that tear and slide underfoot. D&D 5e: The mats are Difficult Terrain: every foot of movement costs 1 extra foot. Our addition: a creature that takes the Dash action across them makes a DC 10 Dexterity (Acrobatics) check or falls Prone.

Adventure hooks

  1. A scholar wants live specimens of the white snails that live nowhere else, and offers a fortune. The cave’s air would kill her in an hour, so she wants someone else to go in, carrying her jars and a canary.
  2. Miners digging a new shaft have broken into the sealed cave, and the stink of rotten eggs is pouring into their gallery. The owner wants the hole plugged; a hermit who has always known about the cave wants it left open, and armed his friends.
  3. A hot spring in the valley has started to smell of sulphur and the bathers have stopped coming. The spa’s owner suspects the cave beneath, and he wants it fixed without anyone saying the word cave to his guests.
  4. The floating mats of the cave have started to drift together into one great sheet, and the water scorpions are fleeing up the walls. A pale centipede the length of a man is the only thing still walking on the mats.

What may happen

  1. A thin white skin floats on the warm water like wet paper and tears when touched. Tiny white things crawl on it.
  2. The stink of rotten eggs grows strong, then suddenly vanishes.
  3. A pale water bug, blind, its forelegs folded like a mantis, hunts in the shallows.
  4. A pocket of air above the water where the roof dips down. The lamp burns smaller here.
  5. A scholar sits by the water with jars and a notebook. She has been here three days and has run out of clean water.
  6. White snails no bigger than grains of rice cover a wet wall in their thousands.
  7. Yellow crystals crust a crack where warm water seeps out.
  8. Bubbles rise from the bottom of the water in long strings and burst with a smell of marsh.
  9. A bat lies at the foot of the shaft, perfect, as if asleep. It flew into the bad air and did not fly out.
  10. A long pale centipede, blind, runs over a boot and away along the wall.
  11. Something that is not a mat shifts at the water’s edge, slowly, against the current.
  12. The narrow shaft the party came down was dug, not found: tool marks on the rock, and a broken pick at the bottom.

Creatures

A town here

A city can live down here, fed by what grows below. Build it in Hearth

To bring back

Ooze Portion, Bat Guano, Brutacian Lung, Bugbear Tooth, Deep Gnome Nose, Fire Beetle Glands, Mycelium, Obsidian Powder, Scorpion Venom, Sulfur Pinch, Topaz Powder, Tourmaline Powder, Troglodyte Thumb Nail, Amber Tears, Amethyst Powder, Aquamarine Powder, Black Powder, Bloodstone Powder, Chalcedony Powder, Chitine Silk, Cockatrice Liver, Crystal Powder, Drow Hair, Echo in Stone, Ettercap Mandible, Ettin Toe, Firenewt Bile, Fixed Nitre, Galeb Duhr Soul, Giant Spider Perfect Eye, Jade Powder, Jasper Powder, Jet Powder, Lapis Lazuli Powder, Magnetite Shavings, Malachite Powder, Moonstone Powder, Myconid Spores, Phase Beast Heart, Ruby Powder, Sapphire Powder, Selenite Sliver, Native Sulfur Crystal, Tanzanite Powder, Young Dragon Tonsils, Zircon Powder, Ankheg Antennae, Barghest Fang, Basilisk Throat, Cranium Rat Brain, Cyclops Eye, Drider Tongue, Flumph Hypothalamus, Grell Beak, Kuo-Toa Gill, Mind Flayer Brain, Minotaur Heart, Ogre Liver, Phase Spider Egg, Troll Bone Marrow, Beholder Eye, Deep Lantern Organ, Dragon Tonsils, Gorgon Muzzle, Oblex Slime, Oni Liver, Roper Bile, Shadow Mastodont Eye, Tainted Quaggoth Soul, Aboleth Eye, Adamantine Dragon Tonsils, Alhoon Brain, Amethyst Dragon Tonsils, Iridium Powder, Elder Oblex Slime, Purple Worm Bile, Sapphire Dragon Tonsils, Shadow Dragon Gland, Tarrasque Carapace Piece, Topaz Dragon Tonsils, Tungsten Powder, Wyvern Poison

Elsewhere below

Fungal forest, Doline jungle, Glowing meadow, Bare labyrinth, Lava tubes, Crystal caverns, The abyss

This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at https://www.dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at https://creativecommons.org/licenses/by/4.0/legalcode.

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