Xiaozhai Tiankeng

The Xiaozhai Tiankeng, also known as the Heavenly Pit, is the world's deepest sinkhole. It is located in Chongqing Municipality.

The Xiaozhai Tiankeng is 626 metres long, 537 metres wide, and between 511 to 662 metres deep, with vertical walls. Its volume is 119,349,000 m³ and the area of its opening is 274,000 m². This material has been dissolved and carried away by the river. The sinkhole is a doubly nested structure—the upper bowl is 320 metres deep, the lower bowl is 342 metres deep, and the two bowls are on average 257 to 268 m across. Between both these steps is a sloping ledge, formed due to soil trapped in the limestone. In the rainy season, a waterfall can be seen at the mouth of the sinkhole.

The Xiaozhai Tiankeng was discovered by specialists in 1994 during a search for a new exploration site for British cavers in the China Caves Project. In fact, it was well known to local people since ancient times. Xiaozhai is the name of an abandoned village nearby and literally means "little village", and "Tiankeng" means Heavenly Pit, a unique regional name for sinkholes in China. A 2,800 step stair has been constructed in order to facilitate tourism.

The Tiankeng formed over the Difeng cave, which in turn had been formed by a powerful underground river, which still flows underneath the sinkhole. The underground river starts in the Tianjing fissure gorge and reaches a vertical cliff above the Migong River, forming a 46 metre high waterfall. The length of this underground river is approximately 8.5 km and during these 8.5 kilometres it falls 364 metres. The median annual flow of this river is 8.77 m³ per second, but its flowrate can reach 174 m³/s. Both the river and Difeng Cave had been explored and mapped by China Caves Project in 1994