Xiaozhai Tiankeng Heavenly Pit, China

The Xiaozhai Tiankeng or Heavenly Pit is both the deepest and largest sinkhole in the world with 626 m length, 537 m width, 511-662 m depth with vertical walls. Its volume is 120 million m³. The sinkhole is a doubly nested structure—the upper bowl is 320 m deep, the lower bowl is 342 m deep, and the two bowls are 257-268 m across.

Known only by locals, it was only "discovered" by the outside world in 1994. The region was added to the UNESCO World Heritage list in 2007. It is home to a thriving ecosystem of some 1285 plant and animal species – including the rare gingko and clouded leopard.

Despite attempting to map the underground river five times in a 10-year period, speleologists found the gushing torrent too difficult to navigate and the team never succeeded.