Located in China's island province of Hainan, Sansha City is the country's southernmost prefecture-level city. It is composed of the Xisha, Nansha and Zhongsha islands and the surrounding sea areas, covering over two million square kilometers. Yet with the land area of only 20 square kilometers, it is the least populated city in the country.

Sitting in the tropical oceanic monsoon climate, the city has more than 280 islands, sandbars and reefs. The sea has an average visibility of more than 30 meters deep.

Yongle Blue Hole, or Yongle Dragon Hole, in the Xisha Islands, is the world's deepest blue hole. Plunging over 300 meters down, it is known as the pupil of the sea. Once a limestone cave that formed millions of years ago, it was then filled with sea water from the melting icebergs.

There are abundant wildlife in the area, including over half of the country's coral species and about 2,000 fish species, which offer rich diets for dozens of seabird species that are counted in 100,000.

Looking down from above, there are separated islands that interconnect beneath the sea, hence its nickname, the Seven Connected Islets. There is a huge reef that forms a common foundation, holding the islets together.

Most of the islets here are made up of coral remains. Coral secretes limestone, which becomes their living shells. They gather together to grow and multiply very slowly, at a rate of only about 2.5 centimeters a year.

(Cover image a still from the video)

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