Based on the trope of classic science fiction where a party finds a hidden corner of the planet with a fabulous ancient civilization, or creatures which were thought to be extinct that are still thriving.
Named after The Lost World by Sir Arthur Conan Doyle, this is, naturally, a geographic location off all maps.
They are usually found in remote locations, such as the center of large and barely explored continents (usually Africa), the polar ice caps, or mysterious islands. They are often home to lost civilizations with amazing Lost Technology, or to prehistoric animals that have managed to survive unchanged—aside from the fact that they suddenly find humans delicious. Some Lost Worlds are almost ludicrously dangerous and populated by fearsome monsters, and still others are Magical Lands where All Myths Are True. Prone to being destroyed by volcanic eruptions, floods, quakes, and/or bombs at the end of the book/film/series, with the protagonists barely escaping.