Subterranean

All ground below the surface behaves the same way whatever cut it: dissolved limestone, a drained lava tube, a mine following a seam or a bored tunnel. The temperature holds within a few degrees all year, there is no sky to navigate by, and the passable route is a single path through solid rock. Losing the light is the main danger, and it happens more often than a cave-in.

Regional environment Terrestrial (land)

Subsurface

Crossing it

Navigate: DC 15
Forage: DC 18
Notice: DC 12

There is one route, set by the rock. Light must be brought, or there is none.

In play

No light

Unlit passage is heavily obscured. A creature without darkvision or a light source is Blinded, and cannot take the Search action.
One route

There is no flanking approach a map does not show. A passage can be held by two creatures against any number.
Sound

Noise travels hundreds of feet and arrives from the wrong direction. Perception checks by hearing reveal that a source exists but not where it is.
Bad air

In a dead-end or a flooded system, a pocket of foul air counts as a Contaminated Atmosphere until it is vented or crossed.

Variations

Cave system

Dissolved limestone. Water-cut and branching, often with the stream that cut it still running.
Lava tube

A drained lava conduit, straight and round, and prone to roof collapse.
Deep crust

Miles down. The rock is hot enough to count as Extreme Heat without a cooled suit.
Flooded system

Sumps and anchialine pools. Passable only by diving, and a diver who loses the guide line cannot find the way out.

Worlds it occurs on

City A planet whose surface is entirely built over, with no open ground and all air, water and food imported.
Foundry World A world converted end to end into industrial plant, with an exhaust atmosphere and a day set by shifts.
Ore A world made of one material throughout, with no biosphere and no weather, mined under contract.
Rime A frozen world with a crystalline ice surface, cold enough that most of its atmosphere has condensed onto the ground.
Sentient A world that is alive as a whole, rare enough that the Federation register lists four and disputes two.
Spore A world the Myrrhiam Bloom has covered end to end, leaving a biosphere that acts under the Bloom's coordination.
Terran (Arctic) A rocky world covered in ice for most of the year but habitable, with open water at the margin in summer.
Terran (Arid) A hot, dry rocky world habitable at its edges, with dune seas across the interior.
Terran (Barren) Rock and wasteland with a thin atmosphere or none, habitable but sparsely settled.
Terran (Verdant) A rocky world with liquid water, breathable air and a magnetic field to protect them, rare and the cause of most wars.

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