Practically since sailing was first invented, sailors have reported seeing mysterious floating objects on the horizon. The most famous of these is the "Flying Dutchman," sightings of which likely originated in the 1600s - but persisted into the 20th century. Usually, these sightings had similar details: A ghostly ship appears just over the horizon, appearing to be lit by some kind of unearthly light, usually during a storm. Sailors theorized that this ghost ship was a former Dutch trading vessel doomed to sail the ocean for eternity, and whenever it appeared before living sailors, it was a bad omen. Poems about ghost ships helped spread these legends even further, like Walter Scott's Rokeby or Samuel Taylor Coleridge's The Rime of the Ancient Mariner.

Ghost ships are just one type of unearthly object people have spotted at sea. In 1643, the Jesuit priest Domenico Giardina reported seeing a floating city over the strait of Messina, just over the horizon. In 1810, cartographers Yakov Sannikov and Matvei Gedenschtrom reported seeing a floating land mass among the New Siberian Islands.

In reality, reports of ghost ships, ghost cities, and ghost islands are all examples of a common optical illusion called "Fata Morgana." (The name derives from the character Morgan le Fay, a sorceress from the legend of King Arthur who could create beguiling illusions.) A Fata Morgana is caused by the way our eye perceives light when it passes through spaces with different densities. At the ocean's surface, the water keeps the air relatively cool. Above that layer of cool air is another layer of warmer air. These differences in temperature create atmospheric layers with different densities. When light passes through them, it refracts, or bends. The human eye assumes the light it can see travels in a straight line, so from a distance, refraction can make an object on the water's surface appear as if it's floating above it. Most likely, superstitious sailors probably were seeing real ships. They just looked like they were floating in the air.

Fata Morganas are convincing enough that even though we now know their cause, we can still fall for them. In 2015, residents of Foshan and Jiangxi reported seeing a floating city in the sky. Once again, it was just a mirage.