OK, W.E. here is my entry in the formation contest. Taken late afternoon in the Australian Alps and the hill in front is the location for the famous horse scene in The Man From Snowy River.
The Mir diamond mine in Siberia is the deepest ‘proper’ man-made hole. It’s 525m deep and 1,200m across. There’s a no-fly zone above it because the downdraft created by the hole had caused several helicopters to crash.