Slood

The discovery of slood is said to be one of the basic hallmarks of any noteworthy civilization. It is, apparently, easier to discover than fire, but slightly more difficult to discover than water. There is an Emeritus Professor of Slood Dynamics at Unseen University.

The Special Theory of Slood was discovered by Archchancellor Sloman, a past Archchancellor of the Unseen University. A stained-glass window commemoration this discovery can be seen in the UU Council Chamber.

The gods of the Discworld have often heard the story of a race of people who lived on a blue world in the shape of a sphere, and how they watched massive asteroids slam into a neighboring planet, and then did NOTHING ABOUT IT because that sort of thing only happens in outer space... The gods find this story very amusing, if not very likely, as any race that stupid would have never been able to discovered slood.

Slood is first mentioned in The Last Continent, and the wizards at UU have "...huge pipes for balancing the slood differential across the university..." in Unseen Academicals.