Tenoumer crater

Tenoumer is one of those places that looks drawn with a compass. In the empty north of Mauritania, on a vast plain of Precambrian granite and gneiss, a ring of dark rock almost two kilometres across rises out of the tan and orange desert, so nearly circular that from orbit it seems stamped onto the landscape. The rim stands about 110 metres above the crater floor, and the space agencies that have photographed it describe it as one of the best-preserved impact craters on Earth.

For a long time its origin was argued over. Dark, glassy rock scattered around the crater looked like basalt, and early visitors took Tenoumer for a volcano. Closer study showed that this "lava" was rock melted in an instant by the impact of an object from space, and that shattered basement rock had been thrown out beyond the rim. Today it is regarded as a textbook simple, bowl-shaped impact structure, though the bowl is now partly filled: a layer of sediment some 200 to 300 metres thick lies on the floor, so the crater was once far deeper than it appears.

Its age is still debated. Early dating put it at roughly 21,000 years, which would make it astonishingly young, while a 2016 study of the impact melt arrived at about 1.57 million years. Either way, the crater is a newcomer set in rock hundreds of millions of years older, and the contrast is part of its fascination: a fresh wound on one of the oldest surfaces on the continent.

Standing on the rim, the scale becomes clear. The crest is a jagged ring of broken rock and cliffs with sand drifting in dune ridges against its flanks, and a walk right around it covers about seven kilometres. Expeditions that reach it sometimes camp inside the crater itself, and the low sun of morning and evening, when the ring throws a long shadow across the floor, shows its perfect geometry best.

Getting there

Tenoumer lies deep in the Sahara, about 210 km from the mining town of Zouérat; the nearest airport is Tazadit (OUZ), serving Zouérat. The crater is reached across open desert by vehicle with a driver and supplies, a full day.s drive from Zouérat.