Phantom islands are islands that are believed to exist and appear on maps for a period of time (sometimes centuries), and they are removed after they are proven not to exist (or the general population stops believing that they exist).
... (caturti refers to the fourth case--that is, the dative). Incidentally, "tatpurusha" is a tatpurusha ("this man"--meaning someone's agent), while "caturtitatpurusha" ...