The artists of Mathendous engraved what they could see. Their catalogue reads like an African game reserve: elephants with curved tusks, long-necked giraffes, wild aurochs cattle, and crocodiles. Crocodiles, in a place where today not a drop of surface water survives. These engravings date to around 6000 BC, when the Sahara was not a desert at all but a green expanse of grassland, lakes, and rivers during what scientists call the African Humid Period. The wadi that gives the site its name once ran with water. The people who lived along it recorded the world around them on stone, leaving a gallery that outlasted the climate that made it possible. To walk the valley now is to read a field guide to a vanished ecosystem.

One image has drawn researchers back for decades. Two feline figures face each other, reared on their hindquarters, legs and arms partly outstretched in what looks unmistakably like combat. The carving is technically extraordinary, the bodies deeply outlined and carefully polished, the eyes rendered as drilled cups. Whoever made it was no amateur. Look closely at the left-hand figure and a second story appears: a small, delicately engraved woman scratched into the polished body of the cat, added by a different hand at a different time. The meaning is lost to us. A myth, a ritual, a contest between spirits, a scene of play. The artists left no key. The power of the image survives anyway, the way great art tends to.

Mathendous sits within the Messak Settafet, a vast sandstone plateau running northeast to southwest near where Libya meets Algeria and Niger. Wadis fan out from it like cracks in old glaze, their flanking cliffs dense with petroglyphs. The site is remote and hard to reach, which has helped preserve it but has also left it vulnerable to oil exploration and vandalism across this corner of the Sahara. Recognition has come in unlikely forms. In 1978, Libya's postal authority issued a set of five stamps reproducing the Mathendous engravings, fixing images carved by Neolithic hunters into the everyday machinery of mid-twentieth-century mail. A herd of stone elephants, miniaturized and gummed, crossing the world on envelopes.

Mathendous is not a single artwork but an accumulation, the work of many hands across a very long time. The engravers returned to these cliffs over centuries, adding to what earlier carvers had made, sometimes cutting new figures over old ones. The styles shift across the rock: some animals rendered in confident naturalistic outline, others more schematic, the techniques changing as the generations did. It is closer to a palimpsest than a painting, a surface written and rewritten. Among the beasts there are human figures too, and scenes whose purpose can only be guessed, perhaps hunting magic, perhaps ritual, perhaps simply the human urge to mark a place as ours. The valley was a gallery that stayed open for thousands of years.

The engravings at Mathendous are not decoration. They are testimony. They tell us the Sahara was once habitable, that people thrived where survival now seems impossible, and that those people looked hard at the living world and wanted to keep a record of it. The greening and re-drying of North Africa is one of the largest environmental shifts the human story has witnessed, and the artists of this wadi lived through its beginning. Their elephants and giraffes are a warning written long before anyone could read it: landscapes change, sometimes completely, and the creatures that fill them can disappear. The cliffs remember what the desert forgot.