Ngorongoro Crater is the collapsed top of a volcano in northern Tanzania, a bowl nineteen kilometres wide with walls 600 metres high, and on its grassy floor live some 25,000 large animals that never leave, which makes it the densest concentration of wildlife in Africa. The volcano was once as high as Kilimanjaro and blew itself apart around three million years ago; the floor it left holds grassland, a soda lake, swamps and a patch of forest, all within sight of the rim, and almost every animal of the East African plains is down there: lions, elephants, buffalo, hyenas, hippos in the pools, zebra and wildebeest in thousands, and about thirty black rhino, one of the last places in Tanzania where they are seen in the open. The day starts on the rim, where the lodges look down into the bowl from the forest, and a 4x4 drops 600 metres down the access track through mist until the walls open onto the floor.
The animals here are used to vehicles. On the dawn drive you ease into the open grass toward a pride of lions resting after the night's hunt and they barely lift their heads; the crater's lions are an island population, hemmed in by the walls and rarely breeding with outside prides, and it shows in their genes. At Lake Magadi, the soda lake in the southwest, lesser flamingos feed on the algae and tint the shallows pink when the rains refill it from November to February. Ngorongoro is one of the few protected areas where people are allowed to live inside it: Maasai families graze cattle across the conservation area and bring them down to the crater floor for water and salt, and at a boma near the rim you meet them on land they have shared with lions for generations. The same conservation area holds Olduvai Gorge and Laetoli, where some of the oldest evidence of our ancestors, including footprints 3.6 million years old, was found. Ngorongoro is a volcano that became a zoo without fences, and the animals stay because the walls are enough.