Wikipedia
Boesmansgat, also known in English as "Bushman's Hole", is believed to be the sixth-deepest submerged freshwater cave (or sinkhole) in the world, having been dived to . It is located in the Northern Cape province of South Africa.
Boesmansgat was believed to have first been explored by amateur diver Mike Rathbourne, in 1977. The greatest depth attained was by Nuno Gomes, in 1996. Its altitude of over makes this a particularly challenging dive, requiring a decompression schedule equivalent for a dive to feet at sea level. (Gomes' dive was a close call, as he got stuck in the mud on the bottom of Bushman's Hole for two minutes before escaping.)
On 24 November 2004, Verna van Schaik set the Guinness Woman's World Record for the deepest dive with a dive to .