In the desert wilderness of Namibia, on southern Africa’s western coast, an unusual river lies hidden beneath the scorching sand for most of the year. It is an ephemeral river – a river that flows above ground for only a few days a year. The water that feeds the Hoanib River collects in the mountains of north-western Namibia. Its course cuts through the desert, travelling mainly underground all the way to the Atlantic Ocean where its waters fan out into a lagoon. Only when exceptional rains fall over the mountains does the water in the lagoon rise enough to break through to the sea. The animals that live here have learned to adapt to the harshness of the desert, where both food and water is scarce. But, although the river flows underground, it provides enough moisture close to the surface to support a thin oasis of trees and bushes. When the rains fall over the mountains the gush of water replenishes the Hoanib. The abundance of water flowing over the desert is short-lived, in just a few days it all seeps into the ground and the plants and animals of the desert have to rely once more on their skills to survive in one of the harshest environments on Earth.