Olduvai Gorge, ravine and archaeological site, northern Tanzania, in the eastern Serengeti Plain. It is a steep-sided gorge, about 35 km (about 22 mi) long and nearly 90 m (nearly 300 ft) deep. The gorge was created by earthquake activity, and the deposits exposed in its walls were formed in an ancient lake bed and are more than 2 million years old. Stone tools, building sites, and bones of early hominids were uncovered here by Louis S. B. Leakey, Mary Leakey, and others.