Trou au Natron

Trou au Natron is a vast volcanic pit in the heart of the Sahara, its dark walls falling up to a kilometre to a floor dusted white with salt. The French name means "hole of natron"; in the Teda language of the Tibesti it is Doon Orei, "the big hole". The caldera lies at around 2,450 metres in the Tibesti Mountains of northern Chad, just south-east of the volcano Toussidé, and measures some 6 to 8 km across in an irregular outline. Its edge cuts into the neighbouring Yirrigue caldera.

The white crust that gives the place its name is made of carbonate salts. Small vents and hot springs on the floor release steam and mineral-laden water; in the desert heat the water evaporates and the minerals stay behind, building the pale layer sometimes called the Tibesti Soda Lake. Four smaller volcanic cones of scoria and tuff rise from the floor. The ragged shape of the rim suggests that the pit was blasted out by a series of enormous explosions, each deepening the hole, with blocks of debris thrown up to 10 km away. The volcano is considered extinct and its last eruption is unknown; a Pleistocene age has been proposed, and it is counted among the youngest landforms of the Tibesti.

The caldera also preserves a record of a wetter Sahara. Thick layers of fossil freshwater snails and diatoms on its slopes and floor show that a deep lake once filled the basin, perhaps as much as 500 metres deep around the end of the last glacial period. Radiocarbon dates from these deposits point to roughly 14,500 to 15,000 years ago, when this desert crater held standing water.

Getting there

Trou au Natron is among the most remote sites in Africa. The nearest sizeable town is Faya-Largeau, about 430 km away across the desert, and there is no road to the rim. The closest airport in a straight line, Sabha in Libya, lies across the border some 700 km off; within Chad the main international gateway is N'Djamena International Airport (NDJ), roughly 990 km to the south. The caldera is reached by desert expedition through the Tibesti.