The wizards of Unseen University are again called upon to defend their creation, Roundworld, this time in a courtroom--where its very existence hangs in the balance. The Omnians fervently believe that the world is round, not flat, and view the discovery of Roundworld as a vindication of their faith. To leave this artifact in the hands of the wizards would be unacceptable. Not only do the academics hold that Discworld is flat, but by creating the Roundworld universe, they have elevated themselves to the level of gods. Ankh-Morpork's venerable tyrant Lord Vetinari agrees to a tribunal, where the wizards Ridcully, Rincewind, and Ponder Stibbons can present their case--with key assistance from a Roundworld librarian named Marjorie Daw. JUDGMENT DAY weaves together explorations of such Earthly topics as big science, creation, subatomic particles, the existence of dark matter, and the psychology of belief--a treat for Discworld fans and readers of popular science alike.