Southeast of Toussidé lies the wide caldera Trou au Natron (also known as Doon or Doon Orei). Its high rim is cut into sequences of lavas and older volcanic cones. In places it is almost vertical. Inside of Trou au Natron are four recent basaltic volcanic cones, the most remarkable of which is the high Moussosomi, which has erupted a lava flow. Three of these cones are deeply eroded. Cones are also located outside of the Trou au Natron, and their eruption products have in part flowed into the crater. Like Toussidé, the position of Trou au Natron appears to be controlled by the ring fault of the pre-Toussidé caldera. A salty swamp lies within Trou au Natron, whose floor is in part covered by evaporites, mainly sodium sulfate.