According to the plans, the facility will fill completely with lava in two minutes and thirty seconds, starting from the exits and moving towards the center of the dungeon.  However, the entire room the lever is in will seal off, while the facility will explode like a small volcano five minutes after the lever is pulled, expelling the heavily fortified room.  (the resulting giant cloud of gas will apparently be vented to Fernia, which is also the principal source of the magma in the self destruct system.