A coastline seen at dusk could alter world maps for centuries. In maritime exploration, entire islands emerged from fear, distance, fog, and ambition, entering official navigation charts long before anyone proved they existed. Phantom islands shaped trade routes, naval expeditions, and imperial claims across the Atlantic and Pacific worlds. Cartographers of the Age of Discovery relied on fragmented reports from exhausted sailors navigating uncertain seas. A volcanic cloud became a continent. A mirage became territory. In nautical history, islands such as Hy-Brasil, Sandy Island, and Frisland survived on European maps because institutions trusted repetition more than verification. Royal navies copied earlier atlases, merchants financed voyages toward invisible land, and governments used uncertain geography to reinforce maritime influence. As global empires expanded, cartographic error became embedded within political authority itself. Using historical sea charts, captain journals, and satellite-era maritime records, this work traces how mistaken observations persisted through centuries of mapmaking. The book follows the transition from hand-drawn navigation systems to modern geospatial science, revealing how technological limits shaped collective belief. Beneath every false island stood a deeper question about certainty, empire, and the human need to complete unknown spaces. The disappearance of phantom islands marked more than scientific correction. It closed a long European chapter in which imagination and geography could no longer be separated at sea.