Guelta d'Archei is an oasis, or rather a guelta, in the heart of the Sahara desert. A guelta is a peculiar type of wetland, typical of desert regions, formed when underground water in lowland depressions spills to the surface and creates permanent pools and reservoirs. Guelta d'Archei is located in the Ennedi Plateau, in north-eastern Chad, hidden behind a canyon. Its waters sheltered by the towering sandstone cliffs typical of this region. Everyday, hundreds of camels are herded into the knee deep water of the guelta by passing caravans for them to drink and rest. Dung from thousands of camels excreted over hundreds of years have turned the water black.