The main feature of the Ngorongoro Conservation Area is the Ngorongoro Crater, the world's largest inactive, intact and unfilled volcanic caldera. The crater, which formed when a large volcano erupted 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 (2.8 to 3.6 miles) high. The crater floor is 1,800 metres (5,900 feet) above sea level. The crater was voted by Seven Natural Wonders as one of the Seven Natural Wonders of Africa in Arusha