The Hollow Earth hypothesis proposes that the planet Earth either is entirely hollow or otherwise contains a substantial interior space. The scientific community has dismissed the notion since at least the late 18th century. The concept of a hollow Earth recurs many times in folklore and as the premise for subterranean fiction, a subgenre of adventure fiction (A Journey to the Center of the Earth, At the Earth's Core). It is also featured in some present-day pseudoscientific and conspiracy theories.

-Wikipedia