really? looks like 50/50 to me, and the biggest ones are certainly surface ones:
http://en.wikipedia.org/wiki/List_of_coal_mines_in_the_Unite...
http://en.wikipedia.org/wiki/List_of_coal_mines_in_the_Unite...
Yes, really. The claim, which I disputed, was that open pit was required. Showing 50/50 (open/closed) disproves that claim.