Guelta d’Archei

Crocodiles left behind when the Sahara dried, in one black pool

A guelta is a desert pool or oasis, and the Guelta d'Archei in north-eastern Chad is one of the most famous in the Sahara. It lies in the Ennedi Plateau, a sandstone massif sculpted by water and wind into canyons, cliffs, natural arches and pinnacles, about 25 miles south-east of the town of Fada. In the largest canyons water is always present, and it sustains plants, animals and people in the middle of the desert.

For centuries caravans have stopped here to water their camels, and every day hundreds of them still come down into the steep-walled canyon to drink, their grunts and bellows echoing off the rock. The mostly knee-deep water has turned black from the dung they leave behind, and that fertilised water feeds algae, which feed fish, which feed the guelta's most remarkable residents.

Those residents are crocodiles. A small group of West African crocodiles, long confused with the Nile crocodile, lives in the pool, one of the last colonies left in the Sahara. Middle Holocene remains and the rock paintings on the cliffs above show that the species once thrived across most of what is now desert, and along rivers and swamps near the southern Mediterranean coast. As the Sahara dried and people hunted them, the populations died out one by one, and the only other known colony, on the Tagant Plateau in Mauritania, has probably been gone since 1996. West African crocodiles are smaller and less aggressive than their Nile cousins, though they can still bite. Frogs live in the guelta too, and desert antelope sometimes come down to drink.

The paintings are part of a much larger gallery. The caves, canyons and shelters of the Ennedi hold thousands of painted and carved images, one of the largest collections of rock art in the Sahara, and UNESCO has listed the massif as a World Heritage Site.

Getting there is an expedition. No paved road reaches the guelta; it is about four days by four-wheel drive across the desert from the capital, N'Djamena, or a flight to Fada and an overland trip from there, followed by a few hours on foot into the canyon.