 These dunes are among the highest in the world; many of them are above 200 metres, the highest being the one nicknamed Big Daddy, about 325 metres high, however the highest dune in the Namib Desert, Dune 7, is about 388 metres high.The highest and more stable dunes are partially covered with a relatively rich vegetation, which is mainly watered by a number of underground and ephemeral rivers that seasonally flood the pans, creating marshes that are locally known as "vlei"; when dry, these pans look almost white in color, due to the high concentration of salt.Another relevant source of water for Sossusvlei is the humidity brought by the daily morning fogs that enter the desert from the Atlantic Ocean.