Sansha Yongle Blue Hole, also named "Sansha Yongle Dragon Hole”, in the Xisha Islands in the South China Sea, is the world's deepest blue hole. 

Plunging 300.89 meters down, it is known as the pupil of the sea. Formed millions of years ago, it was once a limestone cave, which was then filled with sea water after the icebergs melted. 

Seen from high above, the blue hole is like an eye’s pupil, so it is known as the pupil of the sea.

Discovered in July 2016 in China's Xisha Islands, Yongle Blue Hole has been confirmed as the world’ s largest, far deeper than the previous record holder, Dean's Blue Hole in the Bahamas (202 meters).