Phantom island

A phantom island is an island that was or is believed to exist, but does not actually exist. Phantom islands have appeared on maps during the entire history of cartography. Legendary islands like Atlantis are not usually considered phantom islands, as their status is mythical, rather than due to errors in exploration or map making.

Causes for reports
Reports of phantom islands have several causes. A common explanation is the misidentification of a genuine island. Pepys Island, for example, was in reality the Falkland Islands. Sometimes fog banks, icebergs or optical illusions are mistaken for real islands. It is less dangerous to steer around nonexistent islands than hit uncharted rocks, so recording islands that might exist makes sense. Sometimes islands that existed in the past have disappeared due to volcanic eruptions, tectonic activity, sea level rise, or erosion. Some islands are just made up, for fraudulent or political reasons.

Phantom islands in pseudohistory
Occasionally phantom islands get a second life in theories claiming that islands or continents were discovered earlier than the date that is commonly accepted. This is especially common for islands in the Northern Atlantic: maps depicting Antillia, Estotiland, Frisland, Bacalhau, Saint Brendan's Island, Brasil and Greater Ireland have all at one time or another been considered evidence for a pre-1492 discovery of the Americas. Occasionally such a theory may be true — after all, it has now been decisively proven that Vinland, which was discovered and named by Vikings, is Newfoundland — but more often than not such theories are way off and ought to be considered pseudohistory.

Gavin Menzies
A well known example is in Gavin Menzies' 1421: The Year China Discovered the World. Menzies uses a 1455 Portuguese map that depicts the islands of "Antillia" and "Satanzes" as evidence that the Chinese reached the Americas in 1421. Menzies identifies these islands as Puerto Rico and Guadaloupe, despite the fact that Puerto Rico runs from west to east and not from north to south like Antillia, that both islands are depicted at most 2000 kilometers away from Europe, half the distance of Puerto Rico and Guadaloupe, and that the relative position of Antillia and Satanzes do not match those of Puerto Rico and Guadalupe.

Aliens!
Even further from reality, Erich von Däniken claims islands depicted on the 1513 Piri Reis map constitute evidence for extraterrestrial visits to the Falkland Islands.

Phantom islands today
Phantom islands are usually considered something of the past, but even today there are islands of which the existence is not satisfactorily defined.

Notable examples
The islands Maria Theresa and Ernest Legouvé, purportedly located south of the Tuamotu islands of French Polynesia, which were spotted in 1843 and 1902, respectively. It is now generally accepted that both islands do not actually exist, but they still appear on some maps. Maria-Theresa (sometimes called Maria Theresa Reef, Tabor Island or Tabor Reef) was allegedly spotted by a New Bedford whaler called Captain Asaph P. Taber and is noted for its appearance in Jules Verne's novels In Search of the Castaways and The Mysterious Island. A 1966 radio broadcast from Maria-Theresa was later revealed to be a hoax. Ernest Legouve Island or Reef is named after the French ship Ernest‑Legouvé, whose captain allegedly sighted it.

Sandy Island aka Sable Island lies between Australia and New Caledonia, or rather it lay there on maps until someone attempted to visit it in 2012. It was first reported by Captain James Cook in 1774, and from the late 19th century was regularly included in maps. In fact, there are comparatively deep waters where it is supposed to lie.

Sannikov Land was allegedly sighted by several expeditions into Russian Arctic and became a rather popular myth in 19th and early 20th centuries there (it even inspired a paleonthological sci-fi novel which was made into a movie by Soviets). Currently there's no consensus whether the island existed, but was destroyed by erosion (which the whole archipelago is currently undergoing), or it was just a mistake or a mirage.

Other examples
Other phantom islands in the same area are Wachusett Reef, Jupiter Reef and Rangitiki Reef. Wachusett Reef appears in the 2005 edition of the National Geographic Atlas of the World. Another example is Kiribati's Filippo Reef, which probably doesn't exist, but is nonetheless mentioned in the same edition of National Geographic's Atlas.

Bermeja "conspiracy"
Perhaps the most controversial contemporary example is Bermeja. Bermeja is supposedly located in the Gulf of Mexico, north of Yucatán, at 22°33′N 91°22′W﻿. It has been reported on maps since the 17th century, but it has not been sighted since the 19th century, does not appear on even the oldest satellite photographs and a survey from the Mexican Marine Secretariat in 1997 did not manage to find the island. The sea around Bermeja's supposed location is 40 meters deep. Bermeja's "disappearance" is controversial, since its existence is relevant for determining the maritime border between Mexico and the United States in the oil-rich Gulf of Mexico. The case has spawned a conspiracy theory in Mexico, which claims that the CIA dynamited the island in the 1970s in an attempt to rob Mexico of part of its oil reserves. This theory has been defended by several Mexican politicians and has been reported in various media outlets.

According to the Mexican government, it is likely that the island disappeared either because of sea level rise or because of tectonic events. Julio Zamora, president of Mexico's Society of Geography, suggested it was a deliberately inserted to stop rival nations using the map. For the Clinton-Zedillo treaty of 2000, which fixed the maritime borders between Mexico and the United States, the island was considered not to exist. If it had existed, the border would have been 100 nautical miles north of its current location.

Not fake but not there
Vordonisi, an island in the Sea of Marmara off the coast of Turkey, vanished in 1010 CE due to an earthquake, but its summit popped up again around 2013 with further seismic rumblings. Graham Island aka Ferdinandea lies off the coast of Sicily and has appeared and disappeared multiple times due to volcanic activity.

Some islands have disappeared due to climate change, including New Moore (or Moor) Island (aka South Talpatti) in the Indian Ocean, subject of a territorial dispute between India and Bangladesh.