Because the shaft is nearly vertical, rocks riding inside shift a lot. To keep them from breaking down the door and raining into the shaft.
history under ground
mine-shafts
An open mine cars waits to be lowered back into Eagle Mine in front of the rust-locked modern mine shaft in the middle of Gilman.
A quick shot to show the mineshaft in context with the smelter. Did I mention the smelter’s stack is unreasonably gigantic?
Looking through the dark door at Shaft 3, when my naked eyes could only make out a staircase lit dimly from above.
Frankie on the White Pine Mine vehicle access shaft. The mine was traditional inside… all room-and-pillar.
Two counterweighted elevators moved men between the surface, mine, and underground mill.
On the left you can see one of the later air shafts for the mine below, which allowed for natural air exchange with the main production areas of the coal mine. That is to say, there were no fans blowing fresh air down below.