Earth's Giant Hole (2012)

Tiankeng are vast holes in the ground hundreds of metres across and deep enough to contain hidden, subterranean forests. Yet just five years ago, they were unknown to science. In one corner of China in the remote Guangxi Province there are more TIankeng than any where else and no one knows why. We follow world-renowned cave geologist, Darryl Granger and his scientific survey team on a daring expedition to the heart of a previously unexplored Tiankeng to unlock its secrets.