Is the deepest 'blue hole' on Earth in the South China Sea?

South China Sea's blue hole calls into question theories about the way underwater sinkholes form.

Explorers of South China Sea’s "Dragon Hole" have declared it the deepest underwater sinkhole on the planet.

A short distance from the disputed Paracel Islands, the watery hole plunges 987 feet deep, surpassing the previous record-holder, Dean’s Blue Hole, in the Bahamas, by more than 300 feet, China's government-run Xinhua News Agency reported.

And this incredibly deep, oceanic sinkhole challenges the common explanation for how blue holes form.

Jim Culter, senior scientist at the Mote Marine Laboratory and Aquarium in Sarasota, Fla., says the South China Sea's Dragon Hole may not actually be the deepest underwater sinkhole on Earth. "There may be deeper ones around that haven’t been observed because they’re not visible on the surface. All the ones we study offshore in Florida you can’t see from the surface," he says in an interview with The Christian Science Monitor.

The massive underwater sinkholes are named blue holes because the depth and the refraction of sunlight makes them appear bluer than the surrounding water, explains Mr. Culter, who has explored the phenomenon in Florida.

These strange structures formed thousands of years ago, when the seafloor was exposed to fresh rainwater, he explains.

"Most sinkholes start when sea level was lower," Culter tells the Monitor. "You get acidic waters that flow through the limestone cracks and crevices and they form these voids. And they can form underwater rivers and chambers like large caves. When these caves erode enough, the ceiling collapses, creating a sinkhole. That's the most common type that people refer to as blue holes."

This explanation easily fits blue holes that are in the 300-foot range, as are most to have been discovered. But the fact that the blue hole in the South China Sea is significantly deeper than where we know sea level has fallen in the past millennia has scientists, like André Droxler, puzzled.

Dr. Droxler, a professor of Earth science at Rice University, has two possible explanations: Either tectonic activity changed the height of the land or a process other than the traditional carbonic acid method dissolved the limestone to form the hole. Either way, this blue hole provides insight into processes that shaped the planet many years ago.

"That's the excitement of these blue holes. They probably contain some fantastic paleo-planet record," Droxler tells the Monitor. "These blue holes are quite interesting because they are deep and quite often there is a lack of oxygen so these sediments are not disturbed by living organisms."

Droxler's own research into the blue hole of Belize provided compelling evidence of an explanation for the collapse of the ancient Mayan civilization. An analysis of minerals in that blue hole revealed that an extreme drought occurred at the same time as the Mayan civilization disintegrated, between 800 and 900 AD.

Xu Zhifei, vice mayor of Sansha City, which funded the exploration project of the blue hole, told the Xinhua News Agency that the city plans to protect and study the blue hole, and the more than 20 varieties of marine organisms already discovered there.

Oxygen doesn't reach the bottom of this vertical cave, say researchers with the Sansha Ship Course Research Institute for Coral Protection who've been exploring the cave for about a year, according to CCTV News. So while scientists will be able to study the rich variety of marine life in the outskirts of the hole, it's less likely that they'll find life at the bottom.

"These habitats are really unique," Emily Hall of the Mote Marine Laboratory in Florida tells the Monitor. "Some organisms are doing really well there and we're trying to figure out why."

And that's proving a puzzlement for scientists, Dr. Hall says. "Why do they like it there so much? Is it just because there’s a little bit more nutrients? But then there’s the confounding effect of a reduced ph."

More acidic waters should make it more difficult for life to thrive, but "this is in a place where the rest of the bottom on the Gulf is just sand but you get to the holes and there's all kinds of sponges, and crustaceans, and lots of fish hanging out here, and sea turtles and sharks. They’re very active areas."

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