The Sansha Yongle Blue Hole, located at China’s Xisha Islands in the South China Sea, is an underwater sinkhole that researchers believe is the largest of its kind on the planet.
The hole, traditionally known in Chinese as a Longdong or “Dragon Hole” (龙洞), is located at 16.31 degrees north latitude and 111.46 degrees east longitude in the Yongle atoll, a major coral reef at the Xisha Islands.
The Yongle Blue Hole is 300.89 meters deep. Its entrance diameter reaches 130 meters and its bottom’s diameter stretches 36 meters. Previously, the 202-meter-deep Dean’s Blue Hole on Long Island in the Bahamas was considered the world’s deepest known blue hole with an underwater entrance.
No connections between the hole and the outside sea waters have been found. The water inside the cave does not flow.
Blue holes are one of the world’s most recognizable natural wonders. They are underwater sinkholes formed by the erosion of carbonate rocks. They appear as dark blue circles of water in the ocean. Blue holes are typically located in low-lying coastal regions, which were once above sea level many thousands of years ago.
The other four deepest blue holes around the world:

Dean’s Blue Hole, Bahamas (202 meters) 

Blue Hole of Dahab, Egypt (130 meters) 
Great Blue Hole, Belize (123 meters)

The Blue Hole of Gozo, Malta (60 meters)