Ngorongoro is a unique crater known as a caldera. A caldera is the after effect of a volcanic collapse, an implosion rather than an explosion. Ngorongoro was formed some two million years ago. The Ngorongoro crater is found within the Ngorongoro Conservation Area Authority and ecologically it covers an area of 8,300 square kilometers with varied terrain and altitudes. These variations result in diverse and distinct habitats from grassy plains to montane forests. The Crater itself measures an average of 18 kilometers in diameter and approximately 700 meters of vertical drop. Apart from being home to 25,000 large animals, it forms part of what is known as the Serengeti-Ngorongoro-Maasai Mara ecosystem. This ecosystem allows for the free movement, the migration, of 1.5 million animals in their continuous search for green pastures. The crater is one of the prime areas to observe the endangered black rhino and black-maned lions.