The Trou au Natron (Soda Hole) is a secondary caldera within the main Emi Koussi caldera, roughly 8 kilometres wide and nearly 1,000 metres deep. Its floor is crusted white and orange with natron (sodium carbonate) and other evaporite minerals deposited by ancient lake beds and hydrothermal activity. The colour contrast between the dark volcanic walls and the pale floor makes it one of the most striking geological sights in Africa.
The natron has been harvested by Toubou nomads for centuries, used for preserving food and for traditional medicine. Looking down into the Trou from the caldera rim is one of the highlights of any Emi Koussi expedition. Descent into the depression is possible but requires additional rope equipment.