A large stone was placed above the air lock (8[SUP]th[/SUP] level) as bait to activate the flooding.
This stone had strange engravings on it to entice any unworthy treasure seekers to pause and take the bait (stone) away for deciphering, thus allowing time for the tunnels and main shaft to fill with water and be destroyed
Physics proves this premise impossible.
This should have been a scene in the old Indiana Jones movies.
Single stone/air lock.........would you care to describe in detail has this would work.Especially since your whole premise depends on it.