Guelta of Archei

The term guelta, from Arabic (القلتة), denotes in geomorphology a temporary or permanent body of water with no visible outflow: a pool remaining after the flood in a wadi bed, or a natural basin hollowed in the rock. These wetlands are fed by subsurface flow — water held in the alluvium — and often by hillside springs. A guelta's permanence depends on its sheltered position: a deep canyon limits evaporation.

Archei, whose name is said to mean 'desolate land' in Arabic, is considered the most important guelta in the Sahara. It is reached on foot, through an increasingly narrow sandstone corridor, until it opens onto a series of basins hemmed between 300-metre walls.

In a remote part of the guelta survives a residual, entirely isolated population of Nile crocodiles — known here as 'desert crocodiles'. They descend directly from the populations that inhabited the Sahara during its three great prehistoric humid periods, cut off from the rest of the species for several millennia. Dwarfed compared with their relatives, they survive in a few hundred cubic metres of water.

Each day hundreds of dromedaries come down the canyon to drink. It is this traffic, and the dung it leaves, that gives the water its ink-black colour.