Olduvai Gorge is an archaeological site located in the eastern Serengeti Plains, in northern Tanzania. The gorge is a very steep sided ravine roughly 30 miles long and 295 feet deep, and it is part of the Great Rift Valley. The erosion here has revealed various layers which contained fossils and other remnants of very early hominids.
From the late 1930's Mary and Louis Leakey began finding stone tools here, and then continued to find extinct primate skulls. In 1959 they found fossilized parts of a skull of a hominid that had never been recorded before.
The Leakeys continued to find other hominids here, including Homo Habilis which dates back about 2 million years. They even found 3 million year old footprints from a human ancestor walking along a riverbank.
A visit to this extraordinary spot is well worth it as you pass between the Ngorongoro Crater and the Serengeti. There is a small museum.