Retreating once more into the concrete corridors, we snuck down a down a narrow side passage which ended in a strange lattice of 2x8's (seismic support?) followed by a corner around which we found this small, white, featureless cement room.

Someone had written "THE VAULT" on the wall, and it was apt. What could this out-of-the-way room have been designed for?

After a moment in this room, we suddenly heard indistinct sounds like someone approaching... we put out our flashlights and crouched motionless in the darkness of The Vault for about 20 minutes.