Fix_Lag

Never meat your heroes.

penetration rate

ahuehuehue

No but really: what does this mean?

For those who don't know, it's probably (I can't verify this, obviously) a similar system to what the French had with the Maginot line: A shaft twice as deep as necessary, with the top half filled with sand, gravel, or any other loose substance that is hard to dig through. When escape or air is needed, a trap door in the middle of the shaft can be opened, dropping the fill into the bottom of the pit and opening the top half of the shaft.

Unfortunately I don't have any pictures on hand to illustrate this with.

You're getting downvoted but you're entirely correct.

That's not what's happening here.