Stones do not float in water—this is a truism. But there is hardly a rule without exception. In fact, some volcanic eruptions produce a very porous type of rock with a density so low that it does float: Pumice. An unusually large amount of it is currently drifting in the Southwest Pacific towards Australia. When it was first sighted in the waters around the island state of Tonga at the beginning of August, it almost formed a coherent layer on the ocean's surface. The "pumice raft" made it into headlines all over the world.