If a place is overmined, it means too much coal or other things have been taken out of the ground there.
Overmined places can be dangerous and can hurt the earth.
Examples:
The overmined land was not safe to walk on.
The overmined area had no more coal.
The overmined ground was full of big holes.
The overmined field looked like a big cheese with holes in it.
The overmined mountain was as empty as a balloon.
The overmined land was sad and broken.