In a desert plain in Tanzania, in the shadow of the Ol Donyo Lengai, sacred volcano of the Masaki, there is a large dune made of ash from an eruption in 1969. “It is carved into shape by the wind. But the beautiful thing is that it moves bodily. The entire dune walks across the desert in a westerly direction at a speed of about 17 metres per year. It retains a crescent shape and creeps along in the direction of the horns. The wind blows sand up the shallower slope. Then, as e