Lake Natron in northern Tanzania is one of the most alkaline of the Rift system. As its waters evaporate in the intense heat, sodium sesquicarbonate, known as trona or natron, solidifies to resemble giant coral heads in brightly coloured water. Visible beyond the lake is Ol doinyo Lengai (the Maasai's Mountain of God), the only active volcano in the Gregory Rift, which forms an important section of the eastern branch of Africa's Great Rift Valley.