Dead Acacia erioloba trees in Deadvlei, a white claypan inside Namib-Naukluft Park in the Namib Desert of Namibia. Deadvlei is surrounded by the highest sand dunes in the world, averaging 350 metres (1,150 ft) high. About 900 years ago, drought caused the dunes to block the Tsauchab river, which in turn killed the trees. However, because the desert is so dry, they do not decompose.Photo: Ikiwaner