The crown jewel in the Serengeti-Ngorongoro ecosystem is a deep, volcanic crater - the largest unflooded and unbroken caldera in the world. About 20 kilometres across, 600 metres deep and 300 square kilometres in area, the Ngorongoro Crater is a breathtaking natural wonder and hosts a dense population of around 25,000 large mammals which live on the crater floor all year round.