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.

Lurking in the black waters is a small group of surviving Nile crocodiles, a vestige of a wetter time when this species once thrived across most of today's Sahara desert and in swamps and rivers along South Mediterranean shores. Guelta d’Archei represents one of the last remaining colonies of the Nile crocodile known in the Sahara today. The crocodiles feed on fish that survive on the algae that thrive on the waters fertilized by camel droppings. Guelta d’Archei is indeed a zoological marvel.

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