The Ngorongoro Crater is the world's largest inactive, intact, and unfilled volcanic caldera. The crater, which formed when a large volcano exploded and collapsed on itself two to three million years ago, is 610 metres (2,000 feet) deep and its floor covers 260 square kilometres (100 square miles). Estimates of the height of the original volcano range from 4,500 to 5,800 metres (14,800 to 19,000 feet) high. The elevation of the crater floor is 1,800 metres (5,900 feet) above sea level.