Trou au Natron, the depression southeast of the volcano, measures approximately in diameter and in depth. During the last glacial maximum or the early-middle Holocene, it was filled with a lake. A number of volcanic cones have developed within Trou au Natron. Fumarolic activity on the peak of Toussidé and geothermal manifestations within Trou au Natron represent signs of volcanic activity at Toussidé.