Known as Hy-Brasil, a mysterious island, that began to appear on maps from 1325 to 1800s that now seems to only exist in Irish myths. It is supposedly always clouded in mist, except for one day every seven years, but even when it does become visible it can never be reached.
Throughout Europe for centuries stories and urban legends have been told about this Irish island. Some of the tales mention it is a promised land of saints, others example it is a paradise where an advanced civilization lives and one captain claimed it was inhabited by large black rabbits. Looking on most of the printed maps that included the island, it was located around 321km west of Ireland. A distinctive geographical feature of the map shows the island being a circle with a river running across it from east to west.
If we study Celtic history we can derive that Hy-Brasil means the High King, and the earliest mention of the island is in 1325 by Genoese cartographer Angelino Dulcet. It was also featured in the Catalan Atlas in 1375 which placed it as two separate islands but with the same name: Illa de Brasil.
In 1436, it was still present, being found on a Venetian map by Andrea Bianco, and even showed up in 1595 on the Ortelius Map of Europe and Europa Mercator Map. The island would show up on different maps over time in slightly different locations. Yet expedition after expedition could never find the fabled island. John Jay Jr departed from Bristol, England in 1480 only to come back empty-handed after searching for months at sea. However, a Spanish diplomat Pedro de Ayala reported back to the Catholic Monarchs of Spain in 1497, that John Cabot (an explorer who ventured to North America) had been part of the men who had found the Hy-Brasil island.
So this implies someone did set foot on this mysterious island at some point! Today no such island features on any maps, though some historians and cartographers have offered a plausible explanation. The Porcupine Bank discovered in 1862, could have at some point been an island than sank below sea level following a catastrophe, and the location is roughly the same.