Danakil Depression, Ethiopia

 
Discs of travertine ring a 12-foot-wide (3.7-meter-wide) hot spring in Ethiopia's Danakil Depression. Also called the Afar Depression, this area is a tectonic triple junction where the spreading mid-ocean ridges forming the Red Sea and the Gulf of Aden emerge on land and meet the East African Rift. Travertine is a volcanically heated, calcium-rich flow from hot springs. Photograph by Carsten Peter

Kimberley Big Hole - South Africa

Apparently the largest ever hand-dug excavation in the world, this 1097 meter deep mine yielded over 3 tons of diamonds before being closed in 1914.

Glory Hole - Monticello Dam, California

A glory hole is used when a dam is at full capacity and water needs to be drained from the reservoir

This is the 'Glory Hole' at Monticello dam, and it's the largest in the world of this type of spillway, its size enabling it to consume 14,400 cubic feet of water every second.

Bingham Canyon Mine, Utah

This is supposedly the largest man-made excavation on earth.  Extraction began in 1863 and still continues today, the pit increasing in size constantly.  In its current state the hole is miles deep and 2.5 miles wide.

Great Blue Hole , Belize

This incredible geographical phenomenon known as a blue hole is situated 60 miles off the mainland of Belize .  There are numerous blue holes around the world, but none as stunning as this one.

Mirny Diamond Mine , Serbia

This one is an absolute beast and holds the title of largest open diamond mine in the world. At 525 meters deep, with a top diameter of 1200 meters, there's even a no-fly zone above the hole due to a few helicopters having been sucked in.

Diavik Mine, Canada

The mine is so huge and the area so remote that it has its own airport with a runway large enough to accommodate a Boeing 737.  It looks equally cool when the surrounding water is frozen.

Sinkhole in Guatemala

This photo is of a sinkhole that occurred early this year in Guatemala. The hole swallowed a dozen homes and killed at least 3 people.