Skipping everything, right next to the tunnel labelled with "Southside" I believe i found a door on a building, opening it reveals another door, that opens slower, another slower door, and so on.
After opening a last door, there is a sign (read it yourself lolol).

I made this discussion not just because it's an easter egg, but because the sign is also an entity, and I don't know what it does.

Maybe someone already found this, I'm too lazy to lead through 4 pages of discussions.

By the way, you have to wait for each door to fully open before opening the next one.