A prehistoric forest has been uncovered at the bottom of a giant sinkhole in South China by cave explorers. This type of sinkhole is also known as Tiankeng, or ‘heavenly pit’ in Chinese.

At 630 feet, the sinkhole is deep enough to hide the Washington Monument and more. The bottom of the pit houses an ancient forest which spans approximately three football fields in length, with trees reaching over 100 feet tall. According to the Chinese government, it is one of 30 enormous sinkholes in the country.

Cave explorers discovered the sinkhole outside Ping’e village in Leye County, South China’s Guangxi Zhuang Autonomous Region. The explorers climbed into the pit, where they found ancient trees and other plant life.

Karst is a type of topography, ideal for geological wonders like the sinkhole in Leye County, created by groundwater dissolving the limestone rock beneath the surface, according to the U.S. Geological Survey. About 20% of the United States is made up of karst landscapes, including attractions such as Carlsbad Caverns in New Mexico and Mammoth Cave in Kentucky.

Approximately 13% of China is covered by karst topography, according to NASA, with the Guangxi region being a prime example of its beauty. According to George Veni, Executive Director of the National Cave and Karst Research Institute Karst, landscapes vary in both shape and size, depending on the surrounding climate.

“In China you have this incredibly visually spectacular karst with enormous sinkholes and giant cave entrances and so forth,” Veni said. “In other parts of the world you walk out on the karst and you really don’t notice anything. Sinkholes might be quite subdued, only a meter or two in diameter. Cave entrances might be very small, so you have to squeeze your way into them.”