Dragon Blue Hole : The World's deepest in South China Sea

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.)

Blue holes get their name due to the apparent blue color of their surface water.
The surface water in some blue holes can be dark, tannin or even muddy, but the deep water below is usually very clear.

Some blue holes, like Dragon Hole, open up to the marine environment, while others are inland.