Deadvlei is a white clay pan located near the more famous salt pan of Sossusvlei, inside the Namib-Naukluft Park in Namibia. Also written DeadVlei or Dead Vlei, its name means "dead marsh" (from English dead, and Afrikaans vlei, a lake or marsh in a valley between the dunes). The pan also is referred to as "Dooie Vlei" which is the (presumably original) fully Afrikaans name. In Google there are many references to the site, its name often being translated erroneously in terms such as "dead valley"; a vlei is not a valley (which in Afrikaans is "vallei"). Nor is the site a valley; the pan is a desiccated vlei.
Dead Vlei has been claimed to be surrounded by the highest sand dunes in the world, the highest reaching 300-400 meters (350m on average, named "Big Daddy" or "Crazy Dune"), which rest on a sandstone terrace. However, the Wikipedia article on dunes contradicts this claim; firstly, there is room for debate on the criteria, such as whether there are higher individual dunes (the article lists at least three higher than "Big Daddy") or whether there are higher "average highest area" dunes elsewhere (the dune article lists one possibly higher area in Africa.)
The clay pan was formed after rainfall, when the Tsauchab river flooded, creating temporary shallow pools where the abundance of water allowed camel thorn trees to grow. When the climate changed, drought hit the area, and sand dunes encroached on the pan, which blocked the river from the area.