Slood was a natural substance that could be discovered by intelligent beings. It was said to be much easier to discover than fire and only slightly harder to discover than water. The Special Theory of Slood was discovered by Archchancellor Sloman, and a stained glass window representing this event was in the meeting room of the Unseen University college council. The Wizard Rincewind held the position of Reader in Slood Dynamics.
Humans on Roundworld have so far been too unintelligent to find it.