Depths

The Depths is a subregion of the Subterranean region that is important to the plot, accessed through a pipe at the bottom of the Filtration System, making it by far the lowest point in the game. There is no threat of Rain, and there are no natural predators. The only threat are the guardians of the area which will only bother the slugcat if the player has not unlocked the maximum Karma level. The only exit from the Depths is downwards, as there is no way to climb the pipe back up.

The Depths (Spoilers ahead)
The Depths is a seemingly abandoned series of temples and shrines. In order to progress through, the slugcat must have unlocked the 10th Karma level, typically acquired by visiting Five Pebbles. Strange, tall beings guard the area, and will quickly kill the slugcat if it does not have a Karma level of 10. Thankfully, some strange device at the entrance will raise the slugcat to its maximum karma level as soon as it enters the compound. Attempting to make one's way past the guardians without having reached maximum karma is not advised, as they have absurdly high health pools and will kill the slugcat almost instantaneously, but with good timing it is possible to blind them with a flashbang fruit just long enough to sneak past.

As the player progresses downwards, the compound gives way to a loose cave system, and the slugcat will eventually reach a pool of a strange liquid, known as the Void Sea. The slugcat cannot drown in this liquid, and swimming down into it will begin the ending sequence.