are dome-shaped cones, while three have calderas. Ngorongoro Volcano (2.5–1.9 Ma) is primarily basaltic trachyandesite. The caldera is fed by the Munge and Oljoro Nyuki Rivers, while the Ngoitokitok hot springs feed into the Goringop swamp. Lake Magadi is a shallow (1.7 m) alkaline lake. Other volcanoes within the complex include Olmoti (2.01–1.79 Ma), Empakaai, Loolmalasin, Sadiman (3.7 Ma), Lemagrut, and Oldeani. The northwest portion of the conservation area consists of the Serengeti Plains, the Salei Plains, the Oldupai Gorge, and the Gol Mountains inselbergs. These inselbergs are part of the Mozambique Belt quartzite and mica schist about (800–500 Ma) in age. Ngorongoro Crater The main feature of the Ngorongoro Conservation Authority is the Ngorongoro Crater, the world's largest inactive, intact and unfilled volcanic caldera. The crater, which formed when a large volcano exploded and collapsed on itself two to three million years ago, is deep and its floor covers . Estimates of the height of the original volcano range from high. The crater floor is above sea level. The crater was voted by Seven Natural Wonders as one of the Seven Natural Wonders of Africa in Arusha, Tanzania, in February 2013. The Ngorongoro volcano was active from about 2.45 to 2 million years ago. Volcanic eruptions like that of Ngorongoro, which resulted in the formation of Ngorongoro Crater in Tanzania, were very common. Similar collapses occurred in the case of Olmoti and Empakaai, but they were much smaller in magnitude and impact. Out of the two recent volcanoes to the northeast of the Empakaai caldera, Kerimasi and Ol Doinyo Lengai, Doinyo Lengai is still active and had major eruptions in 2007 and 2008. Smaller ash eruptions and lava flows continue to slowly fill the current crater. Its name in Maasai means 'Mountain of God'. The Munge Stream drains Olmoti Crater to the north and is the main water source draining into the seasonal salt lake in the center of the crater. This lake is known by two names: Makat as the Maasai called it, meaning salt; and Magadi. The Lerai Stream drains the humid forests to the south of the Crater and feeds the Lerai Forest on the crater floor – when there is enough rain, the Lerai drains into Lake Magadi as well. Extraction of water by lodges and Ngorongoro Conservation Area headquarters reduces the amount of water entering Lerai by around 25%. The other major water source