For more than a century, an island roughly the size of Manhattan appeared in the Coral Sea on atlases, nautical charts, and eventually Google Maps. It was never there. This episode traces Sandy Island back to the whaling ship Velocity, whose master reported heavy breakers and sandy islets in 1876, and to the cartographic principle of marking any reported hazard just in case. The most likely explanation is a pumice raft from an underwater volcano, a floating field of rock that breaks waves like a shoreline and, as a 2004 Tonga eruption showed, can drift 3,000 kilometers through that exact stretch of ocean.

French aerial surveys removed the island in 1974 and Australian soundings of 1,400 to 2,300 meter depths did the same in 1985, but a data entry error in the US World Vector Shoreline database hard-coded it back into digital life. Downstrea ...