Everyone is transferred to a kind of discworld...one that violates the laws of physics, and which shouldn't exist. Oh, and they share it with some kind of horrible hive-minded insectoid race that ultimately wipes out that particular group of humans. They're not the only ones; apparently there are a ton of replica Earths on the discworld, albeit flattened. It's hard to explain; read it.
This is actually an Alderson Disc. Short version: Building one requires science we don't have, but does not violate the laws of physics.