The undergoing 1 is the most common place you'll end up after going through a hole.
Its full of rooms with doors leading to other rooms.
Listening to doors is pointless as you are timed. In Approximately 1:05 time, you will explode.

The main purpose of the undergoing is to fast travel across the map to key locations. (I recommend that you do your own adventuring to figure out where these points are).

The Hole Room is where you'll end up if you jump down the hole near the beginning of the game (after respawning in the forest).

The cinnamon room has a useful hole leading right outside the circus, but it is bugged currently, not allowing you to enter the hole from inside the undergoing. The hole is still functional outside in the form of a big drum with a tear.

(Correction, apparently the room south of the clock door can be opened after 48 locks or so, but you definitely need a autoclicker and need to enter from the museum manhole)

The Undergoing 2 is a sublayer of undergoing 1 except its more confusing and worse. It is full of long dirt hallways instead of rooms with doors.
Most doors that exist in undergoing 2 are locked.

There is a large creature with red glowy eyes in the northern part of the tunnels, approaching it will result in death.

The undergoing 2 has one-way holes to areas that already have holes leading to undergoing 1.
There are also some rooms here that teleport you into a room in undergoing 1.

This map is unfinished because accessing undergoing 2 is tricky.
There is a guarenteed way to enter undergoing 2: Crawling into the washing machine in the fumigated house. However, if you try to do this again after exiting undergoing 2 and returning to the washing machine, you will instead end up in the washing machine room in undergoing 1.

Because some rooms teleport you into undergoing 1, it is very easy to run out of time and blow up unless you know exactly where to go.