A series of underground Tunnels that Hodge is seeing going through before entering the academy. She repeats the same process after leaving Casey in the past. There is no proof that these tunnels actually lead to the academy.
The tunnels (at least in this instance) appear to be driving distance from the army base in Schaumburg, Illinois. The entrance looks likes a small shaft elevator and is guarded by men who are dressed in the same manner as the guards of the academy. Both Lara and the guards put on hazmat suits before entering the elevator. They then take a long ride through a tunnel to what looks like a decontamination chamber. They remove the suits and she steps through a circle of bright light after opening a bolted door which is controlled by a computer in the room.