Underground

Underground is a meta biome in RimWorld that is never generated on the world map.

Summary

A dark expanse of interconnected caves and tunnels.

Underground is a special biome that is never naturally generated on the world map. Instead, it's used as a base for certain pocket maps placed underground, such as the Undercave, the Ancient Stockpiles, and the Insect lair.

As of Version/1.6.4630, all current implementations of this biome have a constant temperature of 15 °C (59 °F).

This biome has no natural fauna, a plant density of 0.2 (comparable to a tundra) with no flora assigned, and blocks animals from wandering in here.

It has a max fish population is 200, yet there are no defined fish to be caught here.

There is a constant Underground weather present on this biome. This "weather" applies a constant cosmetic filter to the map, is considered good, and has no further effects.

Finally, all tiles are under overhead mountain.

Diseases

Underground has a disease MTB of 80 days. The following diseases can occur in the Underground :

Disease	Commonality
Flu	100
Plague	80
Fibrous mechanites	40
Sensory mechanites	40
Gut worms	30
Muscle parasites	30
Animal flu	100
Animal plague	80

Excluding the impossibility of getting organ decay, this list is identical to the diseases available to cold biomes.

Implementation

Undercave

Main article: Undercave

Used by Pit gate incident as home base for the Dreadmeld.

It uses the weather Undercave, whose only effect is special ambient music. All other distinctive characteristics, such as the fleshmass and the temporal nature of this biome, are created during map generation and not part of the definition of the biome itself. That said, the Pit gate is the only way to access it.

Ancient Stockpile

Main article: Landmarks#Stockpile

Insect lair

Main article: insect lair

Analysis

There are no major threats underground, not even infestations, and enemies from above cannot enter an underground map. Therefore, once the threats inside are cleared (excluding the dreadmeld for the undercave), any of the underground maps are an entirely safe place to store pawns, items, buildings, and so on. Items stored underground are still counted towards the surface map's wealth, so underground maps cannot be used as a form of wealth management.

Permanent settlement

It is possible to make an entire colony underground, which can last indefinitely by growing nutrifungus under fungal gravel, or by using a pawn with body mastery. Otherwise, a colony will eventually run out of food, as hydroponics can breakdown and there is no way to renewably obtain components while underground.

Note that none of the endings can be initiated "purely" underground, with Royal Ascent requiring a bedroom on the main map. Also note that mechanoid raiders on the surface will not leave the map even after everything is destroyed, so if there is nothing defending the surface, it will be difficult to escape.

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Biomes

Temperate

Temperate forest • Temperate swamp • Scarlands

Hot

Tropical rainforest • Tropical swamp • Arid shrubland • Desert • Extreme desert • Glowforest • Grassland • Lava field

Cold

Boreal forest • Cold bog • Tundra • Ice sheet • Sea ice • Glacial plain • Orbit

Water

Lake • Ocean

Pocket map

Underground • Labyrinth • Metal hell • Undercave • Insect lair