Scientists Explain Giant Holes in China

Oct. 18, 2006 — The secrets to the steepest, deepest, most bizarre sinkholes in the world have been worked out by geologists who recently visited the "tiankeng," or "sky holes" in China's Guangxi and Chongquing provinces.

The tiankeng are steep-walled wonders unlike any other sinkholes on Earth, said geologist William White, an emeritus professor at Pennsylvania State University.

"You could fit a couple of Empire State Buildings in and they’d disappear," said White of the more than 2,000-foot-deep wonders.

Yet until recently, no one outside China had even heard of them. "Even the Chinese didn’t know about these until about 15 years ago," said White.

That’s because the tiankeng are in the boonies. Until recently there were no roads to them, just footpaths through the other-worldly conical hills of China's scenic Guilin region.

The first scientific recognition of tiankengs was by Zhu Xuewen, of the Institute of Karst Geology in Guilin. White and his colleagues were part of an international team invited by Zhu in 2005 to study the tiankeng.

"These are pretty unique features," White told Discovery News. "The key factor you need is an underground river."

And not just a trickle, but a pretty good-sized flow of water, he said. As in other cavern-riddled places, it’s the water that dissolves limestone to make caves, the roofs of which can collapse to form sinkholes. Usually, large sinkholes look like valleys with no outlets.

But due to the way the exceptionally thick limestone of the region is fractured, the tiankeng tend to form along vertical cracks — creating the steep walls.

As the caverns collapse over time, the hundreds to millions of cubic yards of rubble on the floor of the cavern is continually dissolved and carried away by the river, White explained. Over eons the cavern opens to the sky and a tiankeng is born.

White is slated to give a report on the tiankeng to fellow geologists on Tuesday, Oct. 24, at the meeting of the Geological Society of America in Philadelphia.

"It’s a hole through which you can see the sky," said Arthur Palmer, a cave expert at the State University of New York at Oneonta. He was on the same tour of the tiankeng as White in 2005 and recalled that you could even walk into a cavern, through the dark and into the bottom of a tiankeng.

In Tennessee there are some sinkholes on a similar scale, said Palmer, but they are not steep-sided and look like normal valleys — until you notice that there’s no outlet. In China's Guilin region, on the other hand, the entire landscape is dominated by tiankeng and conical hills.

"You have the impression someone took a giant cookie cutter to the land," said Palmer. "You are impressed by how abrupt they are. These do tend to be the biggest in the world."

The provincial government is hoping more people make the trip to see the tiankeng. They’ve already built roads and paths so people can walk down into the holes, said White.

"This is kind of hot-off-the-press," said Palmer of how unknown tiankeng are outside of China.

"Although the processes that form tiankengs have been long recognized, the sheer magnitude of these features had not been anticipated," he said. "It is interesting history."