Tanzania's famous Ngorongoro was once a gigantic volcano, perhaps taller than Kilimanjaro. When the volcano collapsed, a vast bowl known as a caldera was formed. Measuring about 18 kilometres across, with a depth of some 250km², Ngorongoro is ringed by an amphitheatre of steep 500 metre-high walls. It is one of the most perfectly formed – and certainly the most spectacular – calderas on the planet. There is an impressive assemblage of wildlife within. There is only one exit and one entry point into the Crater. The world-famous Ngorongoro Crater is a designated World Heritage Site likened to Noah’s Ark in its preservation of diversity.