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).

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.

https://en.wikipedia.org/wiki/Deadvlei