The Xiaozhai Tiankeng (World's biggest sinkhole), Fengjie County, China

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

The Xiaozhai Tiankeng is 626 meters (2,054 ft) long, 537 meters (1,762 ft) wide, and between 511 and 662 meters (1,677–2,172 ft) 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 meters (1,050 ft) deep, the lower bowl is 342 meters (1,122 ft) deep, and the two bowls are on average 257 to 268 m (843–879 ft) 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 has been 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 staircase 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 4-metre (13 ft) high waterfall. The length of this underground river is approximately 8.5 km (5.3 mi) and during these 8.5 kilometres, it falls 364 metres. (1,194 ft) 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 were explored and mapped by China Caves Project in 1994.

1,285 species of plants, including ginkgo, and many rare animals like the clouded leopard have been found in the sinkhole. It contains corridors, halls, craters, collapsed rocks, stone pillars and a type of formation called cave pearls - small, round stones polished smooth by water and deposited in cave crannies, where they sit undisturbed.

A shaft in the large cave hall was found to connect to an underground river, which feeds into the nearby Panyang River.