The Xiaozhai Tiankeng, also known as tiānkēng (天坑), "celestial pits of Xiaozhai", is the deepest chasm in the world. This geological phenomenon, known as a doline, is formed by the erosion of limestone when water seeps into the ground and causes underground cavities to appear. When the ceiling of a cavity becomes too thin, it collapses, forming a depression in the ground. This hole is 192 metres deep, 306 metres long and 150 metres wide. That's a volume of over five million cubic metres! In addition to its size, the Xiaozhai Tiankeng is home to an underground rainforest and extinct plant species.