What term is used to mean a "yet-undiscovered, yet possibly out there civilization"?

A common word for such a civilization is "Atlantean". The word derives from the legend of the lost city of Atlantis, a city that was described by Plato as having sunk beneath the waves of the Atlantic Ocean in early times (about ten thousand years ago). The inhabitants were variously supposed to have had an advanced constitutional civilization.

Why Plato imagined such a thing is unclear. A known history of rapid relative changes in Mediterranean sea and land levels that produced sudden coastal submergences and emergences would certainly have conditioned his mind in that direction. These changes originated, and still do, in volcanic or tectonic activity.

You might want to use the word uncontacted as in uncontacted civilisation. Or perhaps mythical?

Uncontacted (adj): (anthropology, of a tribal society) Completely isolated from other cultures.
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Also see uncontacted peoples