Phantom island

A phantom island is a purported island that appeared on maps for a period of time (sometimes centuries) during recorded history, but was later removed after it was proven not to exist.

Phantom islands usually stem from the reports of early sailors exploring new realms. Some may have been purely mythical, such as the Isle of Demons. Others arose through the mislocation of actual islands, or other errors in geography. For instance, Pepys Island was actually a misidentification of the Falkland Islands. The Baja California Peninsula appears on some early maps as an island but was later discovered to be attached to the mainland of North America; likewise Banks Peninsula off the South Island of New Zealand which was originally called "Banks Island" by Captain James Cook. Thule was perhaps actually discovered in the 4th century BC by the Greek explorer Pytheas but was lost, and then later reidentified by ancient explorers and geographers as either the Shetland Islands, Iceland, Scandinavia, or even as nonexistent.

Other phantom islands are probably due to navigational errors, occasional breakers, misidentification of icebergs, fog banks, pumice rafts from underwater volcanoes or to optical illusions; New South Greenland, observed in the Weddell Sea in 1823 but never again seen, may have been the result of a superior mirage. Even fabrication has been suggested.

...
Wikipedia