Dehydrated Ocean
First mentioned in The Colour of Magic, this "anti-ocean" is located in the Great Nef desert. This is an incredibly dry desert, to the point it has negative rainfall. As a consequence of this, 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. It has very strange fish.