Dragon Hole, also known as Yongle Blue Hole named after the third Ming emperor, is the deepest known blue hole in the world at 300.89 metres (987.2 ft) deep. It is located about 25 kilometres (16 mi) south of Discovery Reef in the Paracel Islands.
Blue holes generate a distinctive blue colour when seen from above. They are characteristically only dozens of metres deep. The local fishermen call it the "eye" of the South China Sea, and believe it is where the Monkey King, depicted in the novel Journey to the West, found his golden cudgel.
Dragon Hole is almost 100 metres (330 ft) deeper than Dean's Blue Hole in the Bahamas. There are several fresh water sinkholes on land that are deeper than Dragon Hole. These include Mexico's Zacatón (335 metres), Pozzo del Merro in Italy (392 metres) and Hranice abyss in the Czech Republic (404 metres).
The Sansha Yongle Blue hole was created around the same time that the surrounding South China Sea islands were created. Blue holes can be formed in a number of ways. The most common is melting ice structures in the surface resulting in large pits where ice once was.
Some cases of blue holes are also formed after a disruption in the surface of the ocean floor by tectonic shifts that result in the settlement of sand and debris.
According to en.wikipedia