On this Thoughty2 video, Arran tells the fascinating tale of Bermeja, an allegedly 80 square kilometers island indicated on many maps, starting in the 16th and 17th centuries. The problem is, it doesn't appear to actually exist. Many theories abound about what could have happened to it or why it would be on so many old maps if it never existed.
I love the idea that it was put there by a mapmaker as a kind of watermark to prove that other cartographers had simply copied his work. Mexico has gone to great lengths to try and prove the existence of the island because it would give them oil drilling rights for 200 miles around it.