The world’s deepest known blue hole has been discovered near the Paracel Islands in the South China Sea.  

Chinese locals have dubbed it the Dragon Hole, and at a depth of 987 feet (300.89 meters), it beats Dean’s Blue Hole, the previous deepest known hole, which is located in the Bahamas and is over 660 feet (201 meters) deep.

The Sansha city government is taking measures to protect and study the Dragon Hole.

Scientists from the Sansha Ship Course Research Institute for Coral Protection used an underwater depth-sensor robot to explore the Dragon Hole. 20 species of fish were discovered in the top 328 feet (100 meters) of the hole, but past that depth, the water in the hole is devoid of oxygen, thus life is highly unlikely to survive there.

According to 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, “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. 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.”

The exact geological and chemical processes for how blue holes form isn’t fully understood by scientists yet. University of Connecticut geoscientist Lisa Park Bousch, who specializes in studying blue hole sediments, believes “chemical reactions at the interface of saltwater and freshwater can create weak acids that eat away at limestone and other carbonates.” Microbial processes may also be responsible for helping to dissolve carbonate rock.  

Blue holes are found both inland and at sea, and other notable examples are located in Egypt’s Red Sea, Australia’s Great Barrier Reef and off the coast of Belize.