Big rocks don't come much more awesome than Ben Amira. Rising 633m out of the desert, it's Africa's biggest monolith, and in size is second only to Australia's Uluru (Ayers Rock). It's clearly visible from the train between Nouâdhibou and Zouérat, but if you have a 4WD it makes a brilliant one-night camping trip from Atâr.
There are actually two granite monoliths. Ben Amira is the largest, with slightly smaller Aïsha to the west. While Ben Amira is more massively spectacular, Aïsha holds a delightful surprise of her own. In December 1999, a symposium of 16 international sculptors was held here to celebrate the millennium, turning many of the boulders at the base of Aïsha into art. The natural shapes of the rocks were reinterpreted as animals, birds, faces and abstract creations. It's a wonderful spot, all the more so for being completely unheralded by its surroundings.
The monoliths are 4km north of the train track between Nouâdibhou and Choûm, at Km 395 (Ben Amira village sits next to the tracks here). The route is sand rather than gravel piste. Aïsha is 5km west of Ben Amira. To find the sculptures, head for the eastern side of Aïsha, where it appears to join a lower mound made of giant 'melted' rocks: the sculptures are here.