Lava tubes

Tunnels left behind when a river of lava crusted over and drained away: long round tubes with smooth walls, ledges where the flow once ran high, and drips of rock frozen on the roof like candle wax. Where the mountain sleeps they are cold and dark, but not here: the rock is warm to the touch, the air smells of sulphur and hot stone, and further in a red light shows where lava still moves. Creatures of fire come up the tubes from the burning places, and smiths and seekers of fire stones go down them. Real ones run for kilometres under Hawaii and Iceland; Kazumura Cave, in Hawaii, is more than sixty kilometres long.

Light
Dim
Going
Slow

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

  • Heat of the rock. Deeper in, the rock is hot and the air dry and burning. Water goes fast. D&D 5e: Near the live lava 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.
  • Live lava. Further in, lava still runs along the bottom of the tube under a thin black skin that looks almost solid. D&D 5e (House rule): Our rule: a creature that enters live lava takes 10d10 Fire damage when it enters and at the start of each of its turns there, and catches fire as under Burning. A creature that ends its turn within 1.5 m (5 ft) of open lava takes 1d10 Fire damage. A crust that looks solid gives way under a creature that fails a DC 13 Dexterity save.
  • Thin floor. Tubes run over tubes, and the floor of one is often the thin roof of another. It rings hollow underfoot. D&D 5e: A creature that breaks through falls 1d4 × 3 m (10 ft), taking 1d6 Bludgeoning damage for every 3 m and landing Prone unless it took no damage. Our addition: a DC 13 Wisdom (Survival) or Intelligence (Nature) check hears the hollow before stepping onto it.
  • Volcanic gas. A heavy gas pools in the low parts of the tube: no smell, no colour, only a lamp that dims and heads that ache. D&D 5e: Where the gas lies the air cannot be breathed. A creature can hold its breath for 1 plus its Constitution modifier minutes (at least 30 seconds), then gains 1 Exhaustion level at the end of each of its turns, and loses them all once it breathes clean air again.
  • Clinker floor. Where the flow broke up as it cooled, the floor is a jumble of sharp, loose clinker that shreds boots. D&D 5e: The clinker is Difficult Terrain: every foot of movement costs 1 extra foot. Our addition: a creature that falls Prone on it takes 1d4 Slashing damage.

Adventure hooks

  1. A smith wants the obsidian that forms where the live lava cools in the lower tubes, for blades that cut like nothing else. Her last two apprentices brought back glass and burns, and one of them did not come back.
  2. The mountain has started to rumble, and the village above the tubes wants to know whether the lava is coming up its old roads. A seer says yes; the tax collector, who has just bought land there, says no.
  3. In a cold side tube there is a floor of ice that never melts, and a cheesemaker keeps his rounds on it. Something has been eating the cheeses, and it has left scorch marks on the ice.
  4. Laughter like crackling wood echoes in the upper tubes at night, and the villagers leave out coals to keep it happy. A new priest has forbidden the custom, and the laughter has stopped, which worries everyone more.

What may happen

  1. Drips of rock hang from the roof like icicles, black and glassy. They snap off at a touch.
  2. A hole where the roof has fallen in lets down a shaft of daylight and a cold wind.
  3. Scorch marks wind along the floor, as if something hot had crawled this way.
  4. In an old side tube far from the live flow, cold air has pooled, and ice lies on the floor all year round.
  5. Lines along the walls, one above the other, show where the lava ran high and where it ran low.
  6. A camp of smiths, their anvil set beside a crack that breathes hot air. They work without charcoal.
  7. The floor trembles with a long low rumble, and dust drifts down from the roof.
  8. A lens of black glass in the wall, smooth as a mirror. It gives back the lamp a little too slowly.
  9. A crack in the floor glows orange, and the air above it shimmers.
  10. Pale roots hang from the roof of a shallow tube, from trees growing on the lava field above.
  11. A skeleton half sunk into the rock, as if the floor had flowed around it.
  12. Crackling laughter echoes down the tube, and small burning shapes dart across the far end.
  13. The walls are glazed where the lava ran, smooth and shining like a pot from the kiln, streaked with silver.
  14. Patches of gold and white speckle the roof and shine in the lamp like spilt paint.
  15. A boulder of rolled lava sits wedged across the tube like a cork, smooth and round. The way goes over it, or not at all.
  16. Under a drip, broken clay jars sit in a row on a ledge. Someone once came down here for water, a long time ago.
  17. Threads of golden glass, fine as hair, have blown into a corner and lie there in a soft tangle.
  18. A man and his daughter knap blades from a seam of black glass by lamplight, a heap of flakes around their knees.
  19. A white crust blooms on a warm wall. A fingertip of it tastes sharp and of iron.
  20. Coins, bones and a little bottle of oil sit on a ledge before a crack that breathes heat. Someone feeds whatever lives below.
  21. Small green crystals glint in the black rock like bottle glass: olivine, which some call the poor man’s emerald.
  22. A round hole in the wall holds the print of bark: a tree stood here when the lava came, and burned out of its stone mould.
  23. Where water seeps onto hot rock, steam fills a side chamber. Someone has set a bench there, and a bucket.
  24. A woman with burnt gloves and a long iron hook fishes something out of a hot crack and drops it, glowing, into a clay pot.
  25. The floor drops away in a frozen cascade of black rock, where the lava once poured from one level to the next.

Creatures

A town here

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

To bring back

Bat Guano, Brutacian Lung, Bugbear Tooth, Deep Gnome Nose, Fire Beetle Glands, Hart's-tongue Fronds, Free-tailed Bat Guano, 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, Oilbird Fat, 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, Edible Bird's Nest, 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, Sulphur cave, 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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