The walls of Ngorongoro Crater do the work of a fence nobody built. It is the collapsed top of a volcano in northern Tanzania, a bowl nineteen kilometres across and 600 metres deep, and the 25,000 large animals on its floor mostly never leave it. You come in over the rim and drop 600 metres down the access track in a 4x4, through forest and morning mist, until the cliffs open onto grassland with animals scattered in every direction around you. Almost every animal of the East African plains is down there: lions, elephants, buffalo, hyenas, hippos in the pools. Wildebeest and zebra graze the floor in long loose columns all year instead of moving on like the herds of the Serengeti, because there is nowhere for them to move on to. About thirty black rhino live here as well, and picking one out far off across the grass is the rarest thing you will see.
Down in the southwest lies Lake Magadi, a shallow alkaline lake where lesser flamingos gather in thousands when the rains refill it and thin out as it shrinks. The animals are used to vehicles. On the dawn drive you can ease into the open grass toward a pride resting after the night's hunt and they barely lift their heads, the cubs sprawled in whatever shade there is. People live here too. Ngorongoro is one of the very few protected areas anywhere that allows it, and Maasai families graze cattle across the conservation area and bring the herds down to the floor for water by day, though nobody settles or farms there. The same conservation area holds Laetoli, where a trail of footprints was pressed into volcanic ash some 3.6 million years ago, proof that our ancestors walked upright long before they had big brains or tools. The walls cut both ways. In 1962 a plague of biting flies crashed the crater's lions to about ten animals. Every lion on the floor today descends from the fifteen or so that were left, and it shows: they are inbred, and fewer of their cubs survive.