An underground river runs through the bottom of the sinkhole to a nearby cave. Provided to China Daily

Picturesque Nam Co in Tibet

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An underground river runs through the bottom of the sinkhole into a nearby cave called Difeng.

However, the path between the "sky hole" and the cave has not been found. To pay a visit to the cave, we had to climb up the sinkhole. The trip was challenging for many visitors.

"It was not easy for us seniors to walk down to the bottom and climb up," says a woman in her 60s surnamed Li from Hubei province. She and her husband took about five hours to make the trip.

But the view is worth the effort, she says.

The nearby Difeng cave has also attracted explorers from China and abroad since it was brought to the world's attention in 1994—though according to the China National Geography, no one has passed all the way through it.

From afar, the cave looks like a narrow gap in the green landscape. The cave is 14 kilometers long and 10 to 30 meters wide, becoming narrower when it goes deeper.

At the bottom of the cave, the width is about 1 to 15 meters. The deepest part of the cave is 229 meters. However, the bottom of the underground river has not been reached.

"The cave remains a mystery, waiting for more explorers," the magazine says.