This is the Great Blue Hole of Belize in Central America, and at 300 metres across and 124 m deep, it's thought to be the largest submarine cave on Earth. It was first discovered by French explorer Jacques Cousteau, and was formed over a period of around 140,000 years when the sea levels were much lower than they are now. As the sea levels rose, a number of limestone caves were flooded over time to create this almost perfect circle. It's now home to several species of fish and reef shark.
