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Lost lands are islands or continents believed by some to have existed during prehistory, but to have since disappeared as a result of catastrophic geological phenomena.

Legends of lost lands often originated as scholarly or scientific theories, only to be picked up by writers and individuals outside the academy. Occult and New Age writers have made use of lost lands, as have subaltern peoples. Phantom islands, as opposed to lost lands, are land masses formerly believed by cartographers to exist in the current historical age, but to have been discredited as a result of expanding geographic knowledge. The classification of lost lands as continents, islands, or other regions is in some cases subjective; for example, Atlantis is variously described as either a "lost island" or a "lost continent". Lost land theories may originate in mythology or philosophy, or in scholarly or scientific theories, such as catastrophic theories of geology.^{[citation needed]}

With the development of plate tectonic simulation software, new lost land has been discovered and confirmed by the scientific community (like Greater Adria in 2019).