Doline jungle

A cave so large that part of its roof has fallen in, letting the sun down onto a jungle on the floor far below. Trees grow up towards the hole, ferns and moss cover every boulder, birds and bats live in the canopy, and clouds form inside the cave and drift out through the opening. The air is warm, wet and loud with insects and dripping water; beyond the edge of the light, the cave goes on in the dark. Such places are real: in Hang Son Doong, in Vietnam, a forest grows under two dolines.

Light
Lit
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, nothing of it grows (too cold)
A few hundred metres down (400 m)
21 °C, it grows
A kilometre down (1 km)
38 °C, it grows

Hear it

Hazards

  • The way down. The only way in is down: a rope from the rim, or a slope of wet boulders under the broken roof. D&D 5e: Climbing costs 1 extra foot for every foot moved, unless the creature has a Climb Speed. On wet rock with few holds, the climber makes a DC 12 Strength (Athletics) check for every 10 m (30 ft); on a failure it falls, taking 1d6 Bludgeoning damage for every 3 m (10 ft) and landing Prone.
  • Cave cloud. Without warning a cloud forms inside the cave and fills the jungle. A hand held out disappears. D&D 5e: Inside the cloud the jungle is Heavily Obscured: a creature trying to see something there has the Blinded condition. At its thinner edges it is Lightly Obscured, and Wisdom (Perception) checks that rely on sight have Disadvantage.
  • Undergrowth on boulders. Roots, fallen trunks and ferns grow over boulders as big as houses. Every few steps there is a hole. D&D 5e: The jungle floor is Difficult Terrain: every foot of movement costs 1 extra foot.
  • Flood. Rain falls on the world above and, an hour later, the river in the cave rises. D&D 5e (House rule): Our rule: when it rains above, the cave river rises 1d4 × 3 m (10 ft) within the hour. A creature caught in the low passages makes a DC 13 Strength (Athletics) check each round to keep its footing, or is swept 1d6 × 3 m (10 ft) downstream and takes 1d6 Bludgeoning damage. A DC 14 Wisdom (Survival) check reads the warning in time: fresh leaves in the water and a rising roar.

Adventure hooks

  1. A rope maker has been hired to hang a ladder down into the great sinkhole, and his ropes keep coming up cut. Something on the jungle floor does not want visitors, and it is clever with a knife.
  2. A naturalist wants to collect the birds that nest under the vault, which no one has drawn. She needs someone to stay in the cave for a whole season, through the floods, keeping her specimens dry.
  3. Every noon, a shaft of sun falls through the hole and moves across the jungle floor. A tribe living below holds that where it touches on the longest day, a chief must be chosen, and this year it fell on a stranger.
  4. The river that flows out of the doline into the dark has started carrying out pale, leafless trees. Something beyond the edge of the light is clearing the forest, and the hunters who camp there want to know what before it reaches them.

What may happen

  1. At noon the sun reaches straight down through the hole, and the jungle steams in a column of gold light.
  2. A cloud forms over the trees and drifts slowly out through the opening, like breath on a cold morning.
  3. At dusk thousands of swifts pour in through the opening, crying, and vanish into the dark parts of the cave.
  4. A tree heavy with a fruit nobody in the party knows, its lower branches stripped by something with hands.
  5. Moss hangs in curtains from the rock, so wet that water runs down it in a steady sheet.
  6. A frayed rope hangs down the doline wall and stops 10 m (30 ft) above the ground. Below it lies a split pack.
  7. Where the jungle ends, the trees grow pale and spindly, leaning towards the light, and then there is only rock and the dark.
  8. A river runs loud through the dark beyond the jungle, and the mist off it keeps every leaf wet.
  9. A hide of woven leaves at the jungle’s edge, facing the dark: someone waits here for whatever comes out to drink.
  10. Something long and many-legged moves through the leaf litter, and all the frogs fall silent.
  11. In a pool at the foot of the wall lie cave pearls, round white stones polished by the drip.
  12. A snake as thick as a man’s thigh lies along a branch over the path, with a large lump halfway down its body.

Creatures

A town here

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

To bring back

Deep Gnome Nose, Mycelium, Chitine Silk, Drow Hair, Myconid Spores, Ankheg Antennae, Barghest Fang, Cranium Rat Brain, Drider Tongue, Flumph Hypothalamus, Grell Beak, Kuo-Toa Gill, Mind Flayer Brain, Beholder Eye, Deep Lantern Organ, Oblex Slime, Oni Liver, Tainted Quaggoth Soul, Aboleth Eye, Adamantine Dragon Tonsils, Alhoon Brain, Elder Oblex Slime, Purple Worm Bile, Sapphire Dragon Tonsils, Tarrasque Carapace Piece

Elsewhere below

Fungal forest, Glowing meadow, Bare labyrinth, Lava tubes, 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.

Open it in Biomes — MGS Toolbox