Subterranean is a late-game region that consists mostly of long, dim tunnels filled with dilapidated trains. It is the final region visited when following the intended path as any Slugcat. The dark pipes of Filtration System connect Subterranean to lower Drainage System and to the caves of the Depths
.
Subterranean spans both sides of the world, connecting to Farm Arrays in the west and Shoreline in the far east. The western side of the region features a steep fissure and a pit overhang which are difficult to scale once Slugcat has dropped down. Subterranean is home to a large number of Centipedes, as well as Miros Birds that patrol the center of the map. Filtration System also contains a high number of Black Lizards, which can easily corner Slugcat in narrow crawlspaces. A subterranean lake in the east of the region contains a unique albino Leviathan, as well as an albino Jetfish when playing as Hunter. Players with fully upgraded Karma can pass through the Depths
.
Filtration System is a dark and dimly lit maze of pipes and shafts connected to Subterranean through four unique tunnels. The lack of illumination makes exploration challenging without a light source. The subregion is also inhabited by a large number of Black Lizards, which can easily corner Slugcat in narrow crawlspaces. Filtration System can also be accessed directly via Drainage System and serves as the only entrance to the Depths.
SPOILER WARNING: Saint This section contains major plot details from Saint. If you have not made significant progress or completed the game as Saint, then read no further!
In Saint's version, Filtration System is lit up with slow-moving lights identical to those in the Depths, and the lower rooms introduce a wavy effect identical to the cave rooms in the Depths. Void Spawn also appear in numerous amounts in these rooms.
Spoilers end here.
Depths
SPOILER WARNING: Ending This section contains major plot details from the end of the game. If you have not yet completed the game on any difficulty, then read no further!
The Depths is the deepest area in the game and can be accessed through a shaft at the bottom of Filtration System. There is no threat of Rain, and no creatures spawn aside from the Guardians to the west of the temple compound. There is no simple way to climb back up the entrance shaft.
The upper area in the Depths contains a series of temple and shrine-like structures. In order to progress beyond this compound, the player must have unlocked the 10th Karma level, typically acquired by visiting Five Pebbles or visiting four Echoes. A small room near the entrance of Depths raises Slugcat's Karma to their current max level. Attempting to move past the Guardians without a maximum karma of 10 results in the Guardians throwing Slugcat back to the start of the room. Repeated attempts cause Slugcat to be flung heavily against the ground, resulting in death. Guardians have extremely high health pools and so cannot be killed conventionally.
As the player progresses downwards, the compound gives way to a loose cave system which eventually opens out into a lake of golden dark liquid, known as the Void Sea. Slugcat cannot drown in this liquid, and swimming down into this lake begins the ending sequence.
Spoilers end here.
Downpour-Exclusive Content
This section contains content exclusive to the Downpour DLC. This content is not available in the base game.
SPOILER WARNING: Saint This section contains major plot details from Saint. If you have not made significant progress or completed the game as Saint, then read no further!
In Saint's campaign, the Depths is almost entirely missing, consumed by the Void Sea. Only the pipe leading to Filtration System and a single platform to stand on remain. The background is also mostly gone, replaced with a gold gradient as a background.
Jumping off the platform before Saint reaches attunement causes Saint to loop around to the top of the room. Maneuvering to the side of the room loops Saint to the other side. If Saint has reached attunement, jumping off causes Saint to fall through a series of empty rooms with increasingly bright backgrounds. After the 1st empty room, Saint becomes perpetually stunned and cannot move. The screen eventually fades to white and a black 10th karma symbol fades in. After a short time, the background fades to black and the symbol fades to white at the same time, and the symbol then fades out. Saint falls into Rubicon at this point.
The shaft in the Filtration System serving as the entrance to the Depths only floods if the More Slugcats Expansion is disabled.
Subterranean contains the Bonus Pearls, a uniquely-coloured pearl which is handled identically to a generic pearl in every other aspect, such as dialogue.
SPOILER WARNING: Ending This section contains major plot details from the end of the game. If you have not yet completed the game on any difficulty, then read no further!
There are multiple unintended ways to bypass the Guardians in Depths. If Remix is enabled and "Vanilla Exploits" is disabled, multiple of the ways are patched out and the exit pipe is blocked below 10 maximum Karma.
With good timing, it is possible to blind them with two Flashbangs for long enough to run past. This was patched out for specifically Hunter.
The Guardians can be bypassed by distracting them with a living creature. This works best with small living creatures such as Hazers or Vulture Grubs that can be swallowed and regurgitated repeatedly, aggravating the Guardians more quickly.
The exit can be reached by manipulating the Guardians to throw Slugcat straight upwards, then throwing objects to reach the pipe.
It is possible to fully despawn the Guardians by interrupting their room loading time. This is considered the simplest and fastest method of the Guardian despawn and is used for any% speedruns. Due to the changes to creature loading in 1.9, it is impossible in that version.
Spitter Spiders are capable of stunning Guardians, who will be unable to interrupt Slugcat while stunned. Enabling Remix patches this, regardless of the "Vanilla Exploits" value.
Guardians may be killed with a Singularity Bomb, allowing Slugcat to pass through unimpeded.
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Slugcat. The dark pipes of Filtration System connect Subterranean to lower 
Drainage System and to the caves of the Depths 
.
Farm Arrays in the west and 
Shoreline in the far east. The western side of the region features a steep fissure and a pit overhang which are difficult to scale once Slugcat has dropped down. Subterranean is home to a large number of 
Centipedes, as well as 
Miros Birds that patrol the center of the map. 
Filtration System also contains a high number of 
Black Lizards, which can easily corner Slugcat in narrow crawlspaces. A subterranean lake in the east of the region contains a unique albino 
Leviathan, as well as an albino 
Jetfish when playing as 
Hunter. Players with fully upgraded Karma can pass through the Depths 
Survivor
Scavenger Storeroom
Subterranean Lake
Dark Magenta Pearl (Subterranean), halfway down the left side of the fissure. This can be reached with a long jump or dislodged with 
Explosives.
Teal Pearl
Two Sprouts, Twelve Brackets, on the right side of the fissure.
SB_E04
SPOILER WARNING: Saint
Saint's version, Filtration System is lit up with slow-moving lights identical to those in the Depths, and the lower rooms introduce a wavy effect identical to the cave rooms in the Depths. Void Spawn also appear in numerous amounts in these rooms.
Saint's version, Filtration System is lit up with slow-moving lights identical to those in the Depths, and the lower rooms introduce a wavy effect identical to the cave rooms in the Depths. Void Spawn also appear in numerous amounts in these rooms.
Guardians to the west of the temple compound. There is no simple way to climb back up the entrance shaft.
Five Pebbles or visiting four 
Echoes. A small room near the entrance of Depths raises 
Maps
Creatures
Overgrown Centipede 
Monster Kelp 
Red Centipede 
Mushroom
Regions
Spears and a 
Grappling Worm or 
White Squidcada. It can also be consistently done as 
Spearmaster.
In one of the western entrances to the Filtration System, rain does not flood the tunnels, allowing Slugcat to stay in one area indefinitely.
Subterranean contains the 
Bonus Pearls, a uniquely-coloured pearl which is handled identically to a generic pearl in every other aspect, such as dialogue.
10 maximum Karma.
Flashbangs for long enough to run past. This was patched out for specifically 
Hunter.
Spitter Spiders are capable of stunning Guardians, who will be unable to interrupt Slugcat while stunned. Enabling Remix patches this, regardless of the "Vanilla Exploits" value.
Singularity Bomb, allowing Slugcat to pass through unimpeded.
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