A new exploration of a legendary blue hole in the South China Sea has found that the underwater feature is the deepest known on Earth.
According to Xinhua News, Dragon Hole, or Longdong, is 987 feet (300.89 meters) deep, far deeper than the previous record holder, Dean's Blue Hole in the Bahamas. (That blue hole measures about 663 feet, or 202 m, deep.) According to Xinhua, local legend holds that Dragon Hole is mentioned in the Ming dynasty novel "Journey to the West," in which a supernatural monkey character gets a magical cudgel from an undersea kingdom ruled by a dragon.
The findings have yet to be confirmed or reviewed by scientists in the field, but if they hold up, the measurements peg Dragon Hole as far deeper than Dean's Blue Hole, said Pete van Hengstum, a marine geologist at Texas A&M University at Galveston, who conducts research on blue holes and sinkholes throughout the Caribbean region. [See Photos of 8 Amazing Sinkholes]
Blue holes are water-filled sinkholes that form in carbonate rock such as limestone. Over long periods of time, the carbonate rock dissolves in the subsurface to form caves or cavities, van Hengstum told Live Science.
"Eventually, the process of dissolution causes the cave to reach very close to the Earth's surface, and if the cave ceiling collapses, a blue hole or sinkhole is formed," he said.
Some blue holes, like Dragon Hole, open up to the marine environment, while others are inland.
It's something of a mystery why blue holes form precisely where they do and what factors influence their development. Chemical reactions at the interface of saltwater and freshwater can create weak acids that eat away at limestone and other carbonates, said Lisa Park Boush, a geoscientist at the University of Connecticut who studies blue-hole sediments in the Bahamas. As a result, rising and falling sea levels can influence when and where blue holes form. [In Photos: Stunning Blue Holes from Around the World]
"There is also a group of researchers looking into microbial processes," Boush told Live Science. In some cases, she said, microbe activity might help to dissolve bedrock and contribute to the formation of blue holes.
In addition to microbes, other organisms also call these jaw-droppingly gorgeous holes home.
"It's interesting to see what actually lives in these blue holes," said Boush, who called the environment of blue holes "cryptic."
Scientists with the Sansha Ship Course Research Institute for Coral Protection in China used an underwater robot and a depth sensor to investigate the mysterious environment of Dragon Hole, which is a well-known feature in Yongle, a coral reef near the Xisha Islands in the South China Sea, according to Xinhua. They found more than 20 marine organisms living in the upper portions of the hole. Below about 328 feet (100 m), the seawater in the blue hole had almost no oxygen, and thus little life, the researchers told Xinhua on July 22.
Even so, diving in blue holes is extremely dangerous, she said.
"One of the reasons why it's very dangerous is because of the limited oxygen," she said. "And sometimes there are even sulfuric waters."
Well-trained divers can make the journey, van Hengstum said. In other cases, researchers park a boat right above a blue hole and send equipment down to measure depth, temperature, oxygenation and other factors. Both Boush and van Hengstum conduct research on the sediments at the bottom of blue holes. These sediments contain information about the past environment and climate change — and sometimes fossils.
The Dragon Hole in the South China Sea probably formed in an environment that's similar to blue holes in the Bahamas, van Hengstum said. Many blue holes currently flooded by seawater in the Bahamas likely originated as sinkholes during a glacial period when ocean levels were lower, but subsequently became flooded after the last ice age, when continental glaciers melted and global sea levels increased, he said.
The Bahamas sit on a big platform of carbonate that's up to 2,000 feet (610 m) thick in places, Boush said. Some of this carbonate is built up by reef organisms like coral, which excrete calcium carbonate as a sort of protective structure. But calcium carbonate comes from many places, Boush said, including calcareous algae (imagine algae with hard, calcium-carbonate skin) and even fish poop.
"Fish eat the coral reefs," Boush said. "They chomp on it — parrotfish, for example. When you go scuba diving, you hear 'click, click, click, click, click,' and that is the parrotfish eating parts of the reef. Well, what goes in goes out again."
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