Lake Natron is a shallow water body that lies in northern Tanzania near the border with Kenya, in the eastern branch of the Great Rift Valley. Depending on the season, it can reach up to 60 km in length and 20 km in width. The intense tectonic activity that characterises the Great Rift Valley has led to Lake Natron being surrounded by huge volcanoes. Those of Sambu and Shompole lie to the north, Gelai to the east, and Lengai and Kerimasi, along with the Embagai Crater, lie to the south. Only Oldoinyo Lengai (or Mountain of God as it is known to the Maasai), remains active. The Lengai is the only volcano in the world to expel whitish lava composed of a strange, sodium-rich form of carbonatite, which, at just 500ºC, is exceptionally cold.


Lake Natron rests at the bottom of a pronounced basin at an altitude of 610 m, surrounded by highlands that reach up 2942 m in the case of the Gelai volcano, or 1800 m in the case of the Serengeti plateau. This topography, along with the dry air masses brought by the monsoon system, gives rise to an exceptionally dry climate. The area’s annual rainfall is barely 500 mm, a figure well superseded by the evaporation rate. This imbalance means that the Natron Basin is one of the driest of the entire Rift. The mean temperature soars to 40ºC in the hottest months, and fresh water can be almost non-existent. The Natron Basin, which includes the Kenyan Lake Magadi to the north, is fed by four permanent watercourses. The main replenisher of Lake Natron, the Ewaso Ngiro River (or ‘Muddy Water’ in Maasai), flows in from the north. The Ngare Sero (or ‘Clean Water’) flows in strongly from the south, while the Rivers Moinik and Peninj (the Anglo-Saxon transcription of the Maasai ‘Pinyini’) flow in from the east. The River Peninj lends its name to the fertile Pleistocene sediments in which so many palaeontological and archaeological discoveries have been made.