Great Nef
The Great Nef, first mentioned in The Colour of Magic is an incredibly dry desert , to the point it has negative rainfall. The Great Nef is 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.
Nef is "fen" (swamp) backwards, an entirely appropriate name for a desert.
Annotation
Is this a Roundworld reference to the Grand Erg in the Sahara?
Appearances
Mentioned in The Colour of Magic