It refers to the phantom island Frisland which was commonly shown on maps of the North Atlantic Ocean during the 16^th and 17^th centuries. Frisland never existed, however, cartographers believed that the island was real because of a map published in 1558 known as the Zeno map. An image of the Zeno map is featured above.

While "The Mysterious Island" is most famous as a work of fiction by Jules Verne (published in 1874), the concept is not based on one single "real" island, but rather inspired by various historical castaway accounts and geographic anomalies. The island in Verne's novel, named "Lincoln Island," is entirely fictional and was described as being in the South Pacific. However, many believe Verne was influenced by the real-life story of Alexander Selkirk, who was stranded on the Juan Fernández Islands off the coast of Chile (which also inspired Robinson Crusoe). Another real-world candidate often associated with the "mysterious" vibe is Henderson Island in the Pitcairn group, which is extremely remote and uninhabited. In a more literal sense, "The Mysterious Island" is a very real themed land at the Tokyo DisneySea theme park in Japan. Built inside a giant artificial volcano (Mount Prometheus), it is considered one of the most immersive environments in the world, featuring attractions based on Jules Verne’s works like 20,000 Leagues Under the Sea and Journey to the Center of the Earth. So, while the geographic island from the book is a legend, the physical manifestation of it exists as a masterpiece of theme park engineering in Tokyo.