Tiankeng means the "The Heavenly Pit", and its sheer sides plummet to a veritable netherworld that"s home to a thriving ecosystem of some 1,285 plant and animal species – including the rare gingko and clouded leopard. Aside from its vast size, the formation is also unique in that it"s a double-nested sinkhole holding two craters joined by a sloping edge. In the rainy season, a waterfall cascades from the mouth of the pit, feeding an underground river and cave network at its base.