Northeast Tanzania exhibits a mountainous terrain and includes Mount Meru, an active volcano, Mount Kilimanjaro, a dormant volcano, and the Usambara and Pare mountain ranges. Kilimanjaro attracts thousands of tourists each year. West of those mountains is the Gregory Rift, which is the eastern arm of the Great Rift Valley. On the floor of the rift are a number of large salt lakes, including Natron in the north, Manyara in the south, and Eyasi in the southwest. The rift also encompasses the Crater Highlands, which includes the Ngorongoro Conservation Area and the Ngorongoro Crater. Just to the south of Lake Natron is Ol Doinyo Lengai with an elevation of , the world's only active volcano to produce natrocarbonatite lava. To the west of the Crater Highlands lies Serengeti National Park, which is famous for its lions, leopards, elephants, rhinoceroses, and buffalo plus the annual migration of millions of white bearded wildebeest. Just to the southeast of the park is Olduvai Gorge, where many of the oldest hominid fossils and artifacts have been found.