Approximately 12 miles in diameter, the Ngorongoro Crater is the largest unflooded and unbroken caldera in the world. It supports a year round population of 25,000 animals. These include zebra, giraffe, wildebeest, gazelle and warthogs. Approximately 20 endangered black rhinos also reside inside the crater. The swamps and forest provide sanctuary for hippos, elephants, waterbuck, baboons and vervet monkeys. Leopards live in the forests as well, but they are solitary creatures, secretive and hard to spot. Other predatory animals such as lions and cheetahs live off the abundant wildlife and hyena roam the crater making their own kills and scavenging from others. One of the earliest signs of mankind is found at nearby Laetoli where hominid footprints from 3.6 million years ago were discovered preserved in volcanic rock.