Ngorongoro Crater is a caldera, the Spanish word for "cauldron," a rimmed oval about 12 miles long and 1500 - 2000 feet deep. An active volcano several million years ago, it is thought to have once stood higher than Mt. Kilimanjaro. According to one theory, during an eruption, it spewed lava from its cone, which emptied the magma chamber underneath, causing the land to lose support. The cone collapsed. Now this caldera, the largest unbroken one in the world, is home to one of the highest concentrations of animals in the world. Within the Crater exists a nearly perfect balance between predator and prey.

Burchell's zebra, Thomson's gazelle, and an enormous resident herd of wildebeest pound across the caldera's volcanic soil, roam over hillock piles of cooled lava, and try to outrun lions, cheetahs, and hyenas that lurk around groves of feathery-leafed acacia trees in the grasslands. At daybreak and sunset, the few remaining indigenous black rhinoceros, along with elephants and cape buffaloes, join the throng to drink from the Munge stream flowing into Lake Magadi, a soda lake in the bottom of the caldera. The lake, which has no natural drainage, grows during the rainy season (April & May) and becomes rich in minerals when the rain ends and water evaporates from its alkaline lake bed.

Ngorongoro Crater is part of a larger section of eastern Africa known as the Ngorongoro Conservation Area (NCA), which includes 3,200 square miles of land. Unlike Africa's national parks, which are closed to full-time human habitation, the NCA is managed to serve the interests of indigenous people as well as wildlife.

The verdant highlands around Ngorongoro are made of restless earth. Volcanic peaks and depressions -- both the process and its results are called "plate tectonics" -- were wrested from the earth by great collisions between the African Plate and the Arabian Plate. The entire Ngorongoro Conservation Area lies along the eastern branch of the Great Rift caused by these collisions.

The area around the Crater rim (approximately 7,200 - 7,600 feet elevation) is heavily forested and inhabited by elephant, buffalo and the elusive bushbuck and bushpig. Birdlife is abundant, and opulent in the case of Livingstone's Turaco and the Bar-tailed Trogan. Within the Conservation Area are soda lakes that host huge flocks of flamingos. Of all the "depressions" in the landscape, the most famous is Olduvai Gorge, where Richard and Mary Leakey discovered the remains of hominids living nearly four million years ago.

The Maasai people, who inhabit the Conservation Area, habitually grazed their cattle in the bottomland, but in recent years have only been allowed to do so with permission from the Ngorongoro Conservation Area Authority (NCAA). The crater floor is reserved primarily for wildlife -- and visitors on safari.