Subterranean

Subterranean is a region consisting mostly of long tram tunnels filled with dilapidated trains. The dark pipes of the Filtration System connect these tunnels to the caves of the Depths. Reaching this region before upgrading max Karma the highest level (10), the slugcat's progression will be stopped by the guardians in the Depths of Subterranean. This is the final region in the game.

Creatures
The following creatures may be found in Subterranean on easy and medium difficulties:
 * Centipedes
 * Guardians (only in Depths)
 * Lizards
 * Black
 * Blue
 * Green
 * Magenta
 * Salamanders
 * Wolf Spiders

Additionally, the following creatures can be found in this region on hard difficulty:
 * Dropwigs
 * Monster kelp
 * Scavengers
 * Miros Birds
 * Worm grass
 * Leviathan (only one, found near Shoreline gate)
 * Spitter Spiders
 * Red Centipedes
 * Cyan Lizards

Echoes

 * Two Sprouts, Twelve Brackets

Region gates
Karma Gates to the following regions may be found:
 * Drainage System (1 Karma Required)
 * Shoreline (2 Karma Required)
 * Farm Arrays (5 Karma Required, but note that the gate cannot generally be reached from the Subterranean side)

Subregions

 * Filtration System
 * Depths

Trivia

 * When entering this region from Farm Arrays, you have to jump off a cliff. This is a one-way trip, but there are several easily missable items of interest on the way down. Down the right side is an echo which will increase your max karma. A crimson colored pearl can be found half-way down the left side off the cliff, that is difficult to reach. You may make a long jump from the top of the cliff or throw explosives to blast the pearl off.
 * Filtration System can only be traversed safely with a light, which can be obtained either through consuming a neuron fly or by trading a scavenger merchant for a glowstone.