Inn-Sewer-Ants: the translation of an Agatean concept brought to Ankh-Morpork by Twoflower. The idea was that a financial institution, upon payment of a yearly "premium", would guarantee reimbursement of loss of property due to fire or other disaster. This must have worked well in the Aurient, but Morporkians soon found the opportunity to reap windfall profits of thousands of percent irresistible. The ensuing chain of events included the Great Fire of Ankh-Morpork, the formation of the Firefighters' Guild, and Charcoal Wednesday.
Blown by the up-draft, the scorched parchment of the Inn-sewer-ants polly-sea floated across the Disc. It came down on the Brown Islands where the natives laughingly worshipped it as a god. For the next seven years they received good rainfall and bumper harvests of crops.
A research team of UU wizards later dispatched to study this event finally concluded; 'it only goes to show'.