[Interesting Times]

My guess is that they're not 'down' 4000 ft -- they may be traveling on an incline so the length of their path is 4000 ft. but they're only ?500-800 from the surface.

4,000 feet is quite possible -- the deepest mine in Canada is 7,200 feet.

[Alberta's Child]

Interesting link.

I was always under the impression that the copper and nickel mines in the Sudbury basin of Ontario are all open-pit mines. Sudbury may very well be the ugliest city in North America, due to the scarred landscape and piles of mine tailings.