An old legend, dating back to at least 1325, describes an island off the West coast of Ireland called Hy-Brasil. Like Atlantis, the island has been impossible to locate. Unlike Atlantis, there are some fairly recent accounts (admittedly, mainly of the "somewhat dubious" variety) of such an island by experienced explorers, plus nautical charts over hundreds of years showing Hy-Brasil's purported location.
The first map depicting Hy-Brasil was created in 1325, then under the name "Brasil". It would change names over the years – including the monikers Brasil, Brazil, Breasil, Hy-Brasil, or O’Brasil – but the intriguing thing about subsequent maps was how they couldn't agree on a location. The island moved around the coast of Ireland somewhat erratically over the centuries, once even popping up just off the coast of North America.
Accounts of the island are stranger still, ranging from sightings of an island shrouded by mist to tales of an island kingdom inhabited by ancient people. One account from 1629 tells how "several seamen have discovered it att sea, as they have sailed on the western coasts of Ireland".
Of these supposed sightings, one man named "Captain Rich" claims to have come "so neere that he discovered a harbour", at which point he began to sail towards it. In a reverse-Desmond-from-Lost, Captain Rich continued to sail at the island as a mist came in, but was never able to make it to shore. The account, even as one of the most grounded of the island, is laden with folklore. It ends with the conclusion it "may be those famous enchanters now inhabitt there, and by their magick skill conceal their iland from forraigners."
Another account – a letter sent by a man in Derry to a friend in England – describes the voyage of one Captain John Nisbet, who found him and his crew stranded on Hy-Brasil and went out to explore. On the first day, they found the island to contain an old castle and yet was completely uninhabited, but after going to sleep that night they woke up and "saw a very ancient grave gentleman, and ten men following him bareheaded (as if his servants) coming towards the shore, where the ship lay".
Dropping all mysterious pretense, the old man threw a feast where he began to tell the crew that "the island was called O'Brazile; that his ancestors were sometimes princes of it, telling them also, that he and several persons of quality, by the malicious diabolical art of a great Negromancer, had been tyrannically shut up in the castle they knockt at yesterday".
Thankfully, the reign of the necromancer was cut short by the sailor's incredibly believable visit. The curse was broken, and others would be able to see the island now. This didn't really pan out, as though there were other accounts of the island, its days as being considered anything other than a myth were numbered. A map made in 1872 would be its final appearance.
There probably was no island to begin with. – phantom islands have been appearing on maps long before, and even after this. These are land masses that appear on maps despite having probably never existed in any physical sense.
One known as Sandy Island survived on maps from 1774 all the way up to Google Earth. It was finally removed after a group of scientists sailed through the alleged island, and declared it non-existent, that it was removed from Google Maps and Earth.
Phantom islands can be the result of myth being incorporated by mapmakers (even Atlantis made its way onto real maps, appearing on one in 1664), mistakes made by explorers (such as Pepys Island, created in 1683 when a captain made calculation errors and mistook one of the Falkland islands for a new one) and mapmakers placing deliberately false islands onto their maps so they would know if their work had been copied. Some could even be the result of sailors encountering illusions, such as the one which produced this "hovering ship" back in 2021.
In the case of Hy-Brasil, there are certainly reasons to suspect it is mainly mythology that birthed the island and kept the legend alive with the help of a few authoritative but incorrect maps. Or, as was suggested in the 1880s, "Brazil might be the present Porcupine Bank" having sunk and turned into a coral reef. Probably – unless you're a big believer in secret islands and ancient curses by necromancers – the former.