A team of Chinese scientists has discovered a giant new sinkhole with a forest at its bottom.
The sinkhole is 630 feet (192 meters) deep, according to the Xinhua news agency, deep enough to just swallow St. Louis' Gateway Arch.
A team of speleologists and spelunkers rappelled into the sinkhole on Friday (May 6), discovering that there are three cave entrances in the chasm, as well as ancient trees 131 feet (40 meters) tall, stretching their branches toward the sunlight that filters through the sinkhole entrance.
"This is cool news," said George Veni, the executive director of the National Cave and Karst Research Institute (NCKRI) in the US, and an international expert on caves.
Veni was not involved in the exploration of the cave, but the organization that was, the Institute of Karst Geology of the China Geological Survey, is NCKRI's sister institute.
A site for sinkholes
The discovery is no surprise, Veni told Live Science, because southern China is home to karst topography, a landscape prone to dramatic sinkholes and otherworldly caves.
Karst landscapes are formed primarily by the dissolution of bedrock, Veni said. Rainwater, which is slightly acidic, picks up carbon dioxide as it runs through the soil, becoming more acidic. It then trickles, rushes and flows through cracks in the bedrock, slowly widening them into tunnels and voids.
Over time, if a cave chamber gets large enough, the ceiling can gradually collapse, opening up huge sinkholes.
"Because of local differences in geology, climate and other factors, the way karst appears at the surface can be dramatically different," he said.
"So in China you have this incredibly visually spectacular karst with enormous sinkholes and giant cave entrances and so forth. 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."
In fact, 25 percent of the United States is karst or pseudokarst, which features caves carved by factors other than dissolution, such as volcanics or wind, Veni said. About 20 percent of the world's landmass is made of one of these two cave-rich landscapes.
The new discovery took place in the Guangxi Zhuang Autonomous Region, near Ping'e village in the county of Leye, according to Xinhua. Guangxi is known for its fabulous karst formations, which range from sinkholes to rock pillars to natural bridges and have earned the region UNESCO world heritage site designation.
Why sinkholes matter
The sinkhole's interior is 1,004 feet (306 meters) long and 492 feet (150 meters) wide, Zhang Yuanhai, a senior engineer with the Institute of Karst Geology, told Xinhua.
The Mandarin word for such enormous sinkholes is "tiankeng", or "heavenly pit", and the bottom of the sinkhole did indeed seem like another world.
Chen Lixin, who led the cave expedition team, told Xinhua that the dense undergrowth on the sinkhole floor was as high as a person's shoulders. Karst caves and sinkholes can provide an oasis for life, Veni said.
"I wouldn't be surprised to know that there are species found in these caves that have never been reported or described by science until now," Lixin said.
In one West Texas cave, Veni said, tropical ferns grow abundantly; the spores of the ferns were apparently carried to the sheltered spot by bats that migrate to South and Central America.
Not only do sinkholes and caves offer refuge for life, they are also a conduit to aquifers, or deep stores of underground water. Karst aquifers provide the sole or primary water source for 700 million people worldwide, Veni said. But they're easily accessed and drained – or polluted.
"Karst aquifers are the only types of aquifers that you can pollute with solid waste," Veni said. "I've pulled car batteries and car bodies and barrels of God-knows-what and bottles of God-knows-what out of the active cave stream."
The new discovery brings the number of sinkholes in Leye County to 30, according to Xinhua. The same researchers have previously discovered dozens of sinkholes in Northwest China's Shaanxi province and a cluster of interconnected sinkholes in Guangxi, China Daily reported.