A barrier placed across a mine passage to prevent ventilation currents from taking the path of least resistance. By blocking off crosscuts and haulage roads, trapdoors force fresh air to travel all the way to the working faces where miners need it. They are designed to open against the air pressure so they naturally blow shut if left unlatched.
Heard on the job
Make sure that trapdoor is shut tight behind you, or the men in the lower panel will lose their fresh air.