A 300-meter-deep, reverse cone-shaped underwater sinkhole found near the Paracel Islands, also known as Xisha, in the South China Sea is now claimed by China’s marine authority as the deepest of its kind worldwide.

Officially named Yongle Dragon Hole, the 300.89-meter blue hole located about 25 kilometres south of Discovery Reef is comparable with the height of Eiffel Tower. It is more than 100 meters deeper than the better-known Dean’s Blue Hole, which is in the Bahamas in the Caribbean.

The almost vertical blue hole measures 130 meters in diameter at its surface entrance and 36 meters at its very bottom, where there is literally no water flow or exchange with the rest of the ocean.

Water in the lower part of a deep blue hole like Dragon Hole is stagnant and anoxic with nearly no water circulation and thus unfavorable for most sea life, but nonetheless can support large colonies of bacteria.

However at least 20 ocean species thrive in the upper section of the hole where there is a unique ecosystem characterized by ample oxygen and nutrients.

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Mystery radiates from the steep-walled, turquoise-colored blue hole that is in striking contrast to the clear shallow water surrounding it. Photo: Xinhua
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A scuba diver surveys a horizontal passage inside the blue hole. Photo: WeChat via Chinese National Geographic magazine
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A scuba diver jumps into the hole to survey it. Photo: China News Service
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With its turquoise color, Yongle Dragon Hole is clearly visible from the air. Photo: Xinhua

Chinese oceanologists and divers are surveying the hole’s halocline, where freshwater meets saltwater and where a corrosive reaction takes place that eats away at the rock wall to form side passages, or horizontal arms that extend from the vertical cave in the sea.

Local fishermen call Dragon Hole the “eye” of the South China Sea, and believe it is where the Monkey King, depicted in the Chinese fork tale Journey to the West, found his gold-banded cudgel.

The vast expanse of water near the hole is under the jurisdiction of the newly-created prefecture-level city of Sansha in China’s southernmost Hainan province, a bid by Beijing to better administer islands and atolls in the combustible part of the ocean occupied by Chinese civilians and military personnel.

The Paracel Islands are also covered by conflicting territorial claims from neighboring countries such as Vietnam that border the South China Sea.

Blue holes develop in a bank or island composed of a carbonate bedrock like limestone or coral reef. They were formed during past ice ages, when the average sea level was some 120 meters lower than at present and these formations experienced rain and chemical weathering erosion common in all limestone-rich terrains; the holes were subsequently submerged at the end of the ice age.