Go through the long concrete corridor, past the wooden maze. Use one of the two last doors, get the door at the back, and enter the multiple-corridor crossing through the next long corridor.

Turn sharp left. The first part of the room is a crusher, so move it! Secret triggers in second half of the room.

Get out, turn left, next room. The center podest is the secret. Afterwards exit the room and go straight across to pick the yellow key up.

Exit yellow key room - some more small corridors have opened - and take the path straight across from #1. You will find a fake exit: the floor drops you into... pretty much of a mess! The secret is the door out again.