Mirny Diamond Mine
Mirny Diamond Mine is an open pit diamond mine located in Mirny, Eastern Siberia, Russia.
The mine is the 2nd largest excavated hole in the world, measuring a diameter of 1200 meters and a depth of 525 meters.
Diamonds were discovered at the site in 1955 which led to the excavation process which started in 1957.
The location of the mine made the excavation incredibly difficult. The 7 month long Siberian winters made the ground hard to penetrate and the extreme weather conditions were even known to burst the tyres of vehicles working in the mine. 
The surface of the mine was closed in 2001 and filled in, in order to allow further mining to continue underground. The underground mines were closed in 2004.  The mine still remains a gaping hole in the ground.
Tours to the mine are available from:

Aerial view of Mirny Diamond Mine