Ngorongoro Crater is a unique kind of crater known as a caldera. It is the aftereffect of a volcanic collapse – an implosion rather than explosion – some two million years ago. Ecologically, Ngorongoro covers an area of 8300 square kilometers with varied terrain and altitudes. This variation results in diverse and distinct habitats from grassy plains to mountain forest. The Crater itself measures 18 kilometers in diameter on average, and has a vertical drop of approximately 700 meters. As well as being home to 25000 large animals it forms part of the Serengeti-Ngorongoro-Masai Mara Ecosystem. This ecosystem allows for the free movement of a further 1.5 million animals in their continuous search for green pastures – a journey known as the migration.