Crystal caverns

A cavern whose walls bristle with crystals, some as long as a ship’s mast, crossing the space like fallen beams. Here they glow faintly from within, a cold white and blue light that sets every face shining, and the slightest sound rings back from them like glass. The air is hot and so wet that sweat never dries, and the creatures that eat stone come here to feed. The real caves of Naica, in Mexico, hold crystals up to twelve metres long, some three hundred metres down at about 45 °C, where people could stay only minutes without cooling suits; those do not glow.

Light
Dim
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

  • Wet heat. The air is as hot as a fever and full of water: sweat does not dry, and the body cannot cool. D&D 5e: In the hot vaults it is over 38 °C (100 °F): a creature without drinkable water makes a Constitution save at the end of each hour, DC 5 and 1 higher each hour after, or gains 1 Exhaustion level. Medium or Heavy armour gives Disadvantage on the save; Resistance or Immunity to Fire damage passes it automatically.
  • Crystal edges. Every crystal has an edge like a knife, and they grow in every direction, floor, walls and roof. D&D 5e (House rule): Our rule: the crystal floor is Difficult Terrain, and a creature that falls Prone on it or is shoved against it takes 1d6 Slashing damage.
  • Off the beams. The only way across is along the crystals themselves, over a floor lost in the glare far below. D&D 5e: A creature that slips takes 1d6 Bludgeoning damage for every 3 m (10 ft) it falls, up to 20d6, and lands Prone unless it took no damage. Our addition: walking a wet beam needs a DC 12 Dexterity (Acrobatics) check for every 10 m (30 ft).
  • Ringing crystals. A shout, or a blade striking a crystal, sets the whole cavern ringing like a struck bell, on and on. D&D 5e (House rule): Our rule: after a loud noise, each creature in the cavern makes a DC 12 Constitution save or has the Deafened condition for 1 minute, and anything that hunts by hearing within a kilometre knows someone is there.

Adventure hooks

  1. A guild of glassmakers wants a perfectly clear crystal beam for a temple window, and has bought the right to saw one out. The cavern rings with every saw stroke, and something deeper has started ringing back.
  2. A fly has been found sealed inside a crystal, and it is not dead. A collector wants the crystal, and the fly would like to be consulted.
  3. A face is carved into one crystal beam, deep inside, where no tool could have reached. The crystal grows a little every year, and the face is getting older.
  4. A miner was found hanging from his cut rope above the crystals, alive and refusing to come down because the black crystals below are whispering to him. He will come up if someone else listens to them first.

What may happen

  1. Every crystal gives back a small copy of the lamp, until the party seems to walk among a hundred lights.
  2. A grinding, crunching sound, regular as chewing, comes from behind a wall of crystal.
  3. A fly is caught inside a crystal, perfect, wings spread, as if it could still fly off.
  4. Old ropes hang from a crystal beam, cut, and below them lies a pick handle bleached white.
  5. A crystal as clear as water, as long as a forearm, has broken off cleanly and lies on a ledge.
  6. A single note hangs in the air, high and steady, with no one making it.
  7. A crack breathes out steam in slow pulses, and the crystals around it shine wet.
  8. In one corner the crystals do not glow. They are black, and warm to the touch.
  9. A pool so full of dissolved stone that new crystals grow overnight on anything left in it.
  10. A face carved into a crystal column, so old that the crystal has grown over it.
  11. The wall ripples like water, a shape of rock rises out of it, looks around, and sinks back.
  12. Some beams end in clean, flat cuts: someone has been harvesting them with a saw.

Creatures

A town here

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

To bring back

Amethyst Dragon Tonsils, Bat Guano, Brutacian Lung, Bugbear Tooth, Deep Gnome Nose, Fire Beetle Glands, Mycelium, Obsidian Powder, Ooze Portion, 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, 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, Sulphur cave, 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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