The world is bounded: from Y -256 at the bottom to Y 1024 at the top, with the sea at Y 0. The
lowest 4 layers are 
Bedrock, which cannot be broken. Everything between the soil
and the bedrock is rock, hollowed by several kinds of caves.
The rocks
Under the soil lie layers of rock, thickest near the surface and ending on a basement rock that fills the rest down to the bedrock (granite unless the province says otherwise). The world is cut into geological provinces about a kilometre across, each with its own stack:
| Province | Layers from the surface down |
|---|---|
| Sedimentary basin | sandstone, limestone, sandstone, limestone, andesite, then granite |
| Limestone plateau | thick limestone, sandstone, granite, then andesite |
| Granite shield | a thin skin of andesite, then granite |
| Volcanic plateau | basalt, andesite, then granite |
| Andesite massif | thick andesite, then granite |
| Slate belt | slate, andesite, then granite |
Layers wander up and down and pinch out in places. Each island draws its provinces from its own table (see Islands and biomes): the Mitan is mostly sedimentary basin, Aube granite shield with slate toward its north, Vepre limestone plateau, the Givre granite, slate and some basalt, Cendree sediments in the west and basalt in the east. Under the sea, the rock of the nearest island runs on for up to 1600 m.
Where the land is humid, pockets of 
Clay lie just under the soil, at most 8
blocks deep.
Rock and ore give nothing by hand: you need a pickaxe. 
Granite and
Basalt need at least a copper one; limestone, sandstone, andesite and slate drop
with a flint one. See Tools and mining.
Kinds of caves
- Tunnels wind through the rock, wider than they are tall, wandering sideways more than up.
- Chambers are rooms 18 to 64 m under the surface, 5 to 11 m in radius, with side lobes and joined to their neighbours by narrower galleries.
- Entrances open on dry ground on hillsides, cliffs and valley floors, a few per square kilometre: the mouth sinks about 7 m over 12 m, then a gallery leads down to the nearest chamber.
- Ravines are narrow canyons open to the sky, 16 to 52 m deep.
- Sinkholes, mostly on limestone plateaus: dolines (a bowl 7 to 16 m in radius with a hole at the bottom 10 to 28 m deep) and shafts, vertical drops of 20 to 80 m into a chamber.
- Great halls start under Y -48 and reach full size under Y -128, with stone pillars and floors 4 blocks thick every 32 blocks, broken in places: several storeys in one cavern.
- Underground lakes lie at the bottom of some halls, between Y -232 and Y -100: discs of water 14 to 36 m in radius and 4 to 9 deep.
What shapes them:
- The rock. Limestone hollows most readily (factor 1.35), then sandstone (1.05), andesite (0.9), basalt and slate (0.85), and granite barely (0.6).
- The island. Vepre is riddled: more tunnels and chambers, two and a half times the entrances, many sinkholes, and halls starting 32 m higher. Aube’s granite is nearly solid, about half the caves of the Mitan. The Givre and Cendree are close to the Mitan.
- Water. No cave opens under or beside the sea, a lake or a river: at least 6 blocks of rock stay between the caves and any water bed, and entrances need dry ground all around their mouth. Water never drains into the caves.
- The surface. Tunnels, chambers and halls keep at least 10 blocks of rock under the ground right above them, but one deep under a plateau may still open in a nearby cliff.
Ores
Ores grow in bodies (veins, seams, lumps) inside their own host rocks, at depths counted down from the surface of the column. A body reaches at most 32 m from its centre.
| Ore | Host rocks | Depth under the surface | Pickaxe |
|---|---|---|---|
 Coal Ore | sandstone, limestone | 2 to 80, flat seams | flint |
 Copper Ore | andesite, granite, basalt, sandstone | 2 to 96, thin veins | flint |
 Flint Nodule | limestone, sandstone | 1 to 120, small lumps | flint |
 Rock Salt | sandstone, limestone, andesite | 60 and deeper, large beds | flint |
 Tin Ore | granite | 4 to 120, small veins | copper |
 Galena Ore | limestone | 8 to 140, masses | copper |
 Quartz Ore | granite | 6 and deeper, small pockets | copper |
 Iron Ore | granite, andesite, basalt, limestone | 6 to 160, lumps | bronze |
 Gold Ore | granite, andesite, basalt | 32 and deeper, below sea level, rare | bronze |
 Magnetite Ore | basalt, granite | 16 and deeper | tier 3 |
Jet Ore lies inside about a third of the coal seams, and a bed of
Fire Clay (dug with a shovel) lies about 2.5 m under more than half of them.
Peat forms wide beds 1 to 4 blocks under the turf of swamps, tundra and taiga,
low on the coasts. 
Bog Iron Ore lies 1 to 5 blocks deep in the soil, peat and
clay of the swamps, and needs at least a copper shovel.- A flint nodule drops a
Flint, sometimes two. - Magnetite needs a tier 3 pickaxe. The best in the game today, bronze, is tier 2: magnetite breaks but gives nothing yet.
Below its tier a block still breaks, slowly, but drops nothing.
Ores by island
Each island has only some of the ores, at its own rate:
- Mitan: coal (rich), iron, flint, rock salt (only here), a little galena, jet, fire clay. No copper, tin or gold.
- Aube: copper, tin and gold (all rich), iron, quartz, magnetite. No coal.
- Vepre: coal, iron, galena and bog iron (all rich), flint, peat, jet, fire clay.
- Givre: iron, copper, gold, quartz, magnetite, peat, bog iron, very little tin.
- Cendree: copper and magnetite (rich), iron, flint, peat, bog iron.
What to do with the ores: see Metalworking.
Light in the dark
Light has two parts, from the sky and from blocks, each from 0 to 15. Deep in a cave the sky gives nothing and the rock is nearly black. Light sources:
| Source | Light | Recipe |
|---|---|---|
 Torch | 14 | 1 stick + 1 fat gives 2, or 1 stick + 1 birch tar gives 4 (by hand) |
 Brazier | 14 | 4 bronze ingots and 1 charcoal (ICI over I.I) |
 Basin of Light | 14 | granite ashlar, bronze ingot, 2 tallow |
 Lantern | 13 | bronze ingot, glass pane, candle |
 Oil Lamp | 11 | fired bowl, fat, plant twine |
 Candle | 10 | 1 tallow + 1 plant twine gives 2 (by hand) |
4 - A torch’s item places a standing torch on a floor or a wall torch against a wall. Standing torches, candles and lamps fall when the block under them is broken.
- A lantern placed against a ceiling hangs from it.
- Holding a torch or a candle lights the area around you as you walk, up to about 9 m for the brightest.
- Within 4 blocks of a placed torch you are warmed and heal three times faster.
Light does not keep the cave dwellers away: they come wherever there is a roof of solid blocks over them, lit or not.
Water, falls and breath
- Underground lakes are up to 9 blocks deep. Under water you lose 6 breath per second out of 100, so about 16 seconds; out of breath you drown at 1 health per second.
- There is no lava in the current game.
- Falls hurt past about three blocks: ten blocks cost about 10 of your 20 health and a little over twenty kill. Shafts, ravines and hall floors are far deeper than that; a ladder, or landing in water, saves you.
Creatures of the dark
They only appear underground, in the dark, at their own depths under the surface:
- Bats, 4 to 40 blocks down: neutral, they hang most of the time. At dusk a colony swarms around its cave, which shows where a cave opens.
- Rats, 2 to 48 blocks down, in packs: they defend their nest, and their bite gives a fever.
- Weavers, 12 to 64 blocks down: giant ceiling spiders, always hostile.
They spin
Web across their lair (a web slows you to a crawl, and theirs root you
for 2 seconds) and drop on prey below; their bite poisons. Cut webs with a knife. - Rock worms, 8 to 64 blocks down, only in Vepre: always hostile, they swim through the stone without breaking it. A pick heard within 40 m draws them, and they lunge out of the wall at the miner, knocking them down.
See Creatures and combat.