From the GPMTCP though the airlock and into the hallway at the top of the stairs. The two wooden doors on the arilock are the only doors still in place, both still move. The closer, interior door has a round hole near the top that was fitted with a flapper valve that would allow excess air pressure in the building to vent out through the airlock using the exhaust valve at the top of the stairs. The pipes in the ceiling are coming from the boiler room and going to the radiators.

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