The Hub

The Hub seems to be a Tunnel, with doors on either side.

Description
The Hub is a concrete tunnel with a series of doors that go to various levels of the backrooms. Currently, one can only access levels 0, 1, and 2. Entities cannot come into The Hub under normal circumstances, so you will find a lot of people there. Certain entities have been known to refer to The Hub as "The Crossroads" on occasion.

Unlike the seemingly infinite backrooms, there is a consistent way of navigating your way to The Hub. From the entrance of Level 1 head straight down the corridor, and stop when you get to the door. Go through the door, then continue until the end of that corridor. Then, double back to the entrance of the level. There should now be a diverging path that goes left or right. Take the left. You will come across another divergence. Take the right. Then, at the next one, take a left, and at the next one, take a right. As soon as you take the right, you will see two doors, labeled A and B. Take the B door. Upon going through that door, you will find yourself in an identical room to the last one. Now, take the A door. This is the door that leads to The Hub. If you get any of these steps wrong, (like taking the wrong turn or not doubling back) you will end up somewhere completely random.

There are also some entrances in Level 2.

Doors 0, 1, and 2, are already pushed open, but are immovable. You cannot close them and the symbols on them are obscured. The tunnel goes on for 6 miles and it expands each level that is discovered, and there are doors to each discovered level ,though many are locked because their Level Key has not been found. The length between each door increases the higher the number gets. Each door has a different insignia on it, looking like a crop circle. The one thing in common between all three of the doors is that they have a metal spike on top of the symbol, and a keyhole below. The keyhole below is a polygonal shape corresponding with the number on the door. But at the end if you can no-clip past the wall you'll find a door that will randomly teleport you to either Level -1, or Level 4.

Entrances and Exits
To Enter, you must be in "Level 1", or "Level 2", and walk through a series of complicated directions mentioned above. To Exit, simply go through any open door, to go to the Level labeled above. If you see a window, you are one step closer to a party! =)