A dune is a hill made of sand. The biggest in Erg Chebbi are around 150 metres tall - higher than a 40-storey building. The wind slowly shifts them around, so the desert is never quite the same shape from one year to the next.
The Sahara looks empty from a distance, but it is full of life. Tiny lizards skitter across the sand. Beetles drink the morning fog. Fennec foxes peek out of their burrows at sunset. Hardy grasses and thorny plants find a way to grow in places that look like nothing should.
Many families who live near the dunes belong to the Amazigh people (also called Berbers), who have called North Africa home for thousands of years. Some still live a nomadic life - moving with their goats, sheep and camels to wherever the grass is growing.
If you stand on top of a tall dune at sunset, the sand glows orange. Then the sky fills with stars - more stars than you can see anywhere with city lights. The Sahara is one of the best stargazing places on the planet.