The Great Nef, was an incredibly dry desert, to the point it has negative rainfall. The Great Nef was home to the Dehydrated Ocean, which consists of water in an uncommon fourth state (it dehydrates, leaving a silvery mildew-like freeflowing sand), and Light Dams, which dam up and save the Discworld's slow-moving, syrup-like light.
The name is a play on The Great Fen, a wetland in Cambridgeshire, England.