Tswaing Crater, South Africa, as viewed from the rim by a group of GPS students and professors during a field trip in September 2000. The floor of the crater is filled with a saline lake previously exploited for the salt and soda ash. Robert DeJong from the Pretoria Cultural History Museum, our guide to Tswaing Crater, talks about the crater's history and role in recent civilization.

Photo courtesy of J. L. Kirschvink