Cave explorers discovered a prehistoric forest at South China’s bottom of a massive sinkhole earlier this month. These sinkholes are also called Tiankeng (or “Heavenly pit”) in Chinese.

The sinkhole, which is 630 feet deep would conceal the Washington Monument. An ancient forest that stretches nearly three football fields long, with trees over 100 feet tall, lies beneath the pit. According to the Chinese government it is one of the 30 largest sinkholes in the county.

Cave explorers discovered the sinkhole in South China’s Guangxi Zihuang Autonomous Province. According to Guangxi news releases, a team of explorers dug into the pit on May 6. They found ancient trees and other plant life.

According to the U.S. Geological Survey, Karst is a topography that allows for geological wonders such as the sinkhole in Leye County. This sinkhole was created when groundwater dissolves the limestone rock below the surface. Around 20% of the United States is composed of karst landscapes. This includes attractions like Mammoth Cave, Kentucky, and Carlsbad Caverns, New Mexico.

According to NASA, karst topography covers approximately 13% of China, with the Guangxi region as a prime example.

According to George Veni (executive director of the National Cave and Karst Research Institute), Karst landscapes can vary in size and shape based on the climate.

Veni stated that China has an incredibly beautiful karst, with huge sinkholes and enormous cave entrances. In other parts of the globe, you can walk on the karst without really noticing anything. Sinkholes can be a mere meter in diameter, so they might seem quite quiet. Cave entrances may be small so you might have to squeeze in order to get into them.

Veni’s Institute is the sister organization of the Chinese Geological Survey team that discovered the new sinkhole. It is the Institute of Karst Geology. Chen Lixin, the Guangxi cave expedition leader, stated that the prehistoric trees found at the bottom are nearly 130 feet tall and the dense forest floor is about shoulder-high. According to the Guangxi news releases.