The Guangxi Zhuang autonomous region is known for its picturesque karst landscape, but the expanse beneath the otherworldly topography of limestone towers and forested spires is just as mesmerizing.

The region, with one of the planet's greatest concentrations of undocumented caves, has become a magnet for cavers and scientists from home and abroad.

During a recent scientific expedition, six French cavers and nine Chinese cavers and scientists explored the karst caves scattered across Guangxi's Donglan county and made a few remarkable discoveries, including a 420-meter-deep sinkhole, traces of an ancient military tunnel and rare plant and animal species.

"We never expected to find such a deep sinkhole, since its entrance is only 100 meters in diameter," said Chen Lixin, a Chinese caver who participated in the expedition.

He said the sinkhole is even deeper, as there is a pool at the bottom, but without diving gear, they couldn't plumb its depths.

The explorers announced their achievements on March 4 at a news conference in the county. Jean Bottazzi, a well-known French caver who has lived in China for several years to explore the country's underground kingdom, said they explored 12 caves with a total length of 16.5 kilometers during a 10-day expedition that started on Feb 26.

The sinkhole is much deeper than common sinkholes found in the area, which are often about 250 meters deep, Bottazzi said.

They also measured a tunnel — 1,992 meters long and with a vertical range of up to 108 meters — and recorded traces of military use that need further research by archaeologists.

Many rare plant and animal species living in the underground caves were also noted.

An all-white fish found in an underground river might be the rare Sinocyclocheilus purpureus, a species of ray-finned fish first found in neighboring Yunnan province. Scientists also found several species of white insects.

Donglan is truly rich in geographic resources, and many caves have unique ecosystems that need further research, Bottazzi said.