Hollow Earth

Our collection on the speculations about the nature and inhabitants of our Hollow Earth (or, if you prefer, Cavern World), the many theories as to the form and nature of that hollowness — including the Concentric Spheres of Symmes, the Concave Earth of Koresh, etc. — and the authors of those theories.

Reading

• Symmes, John Cleves (1878), Symmes, Americus, ed., The Symmes Theory of Concentric Spheres: Demonstrating That the Earth Is Hollow, Habitable Within, And Widely Open About the Poles, Louisville, Ky.: Bradley & Gilbert, https://hatch.kookscience.com/wiki/File:Symmes_-_1878_-_The_Symmes_Theory_of_Concentric_Spheres.pdf 
• Reynolds, Jeremiah N. (1827), Remarks on a Review of Symmes' Theory Which Appeared in the American Quarterly Review, Washington, D.C.: Gales & Seaton, https://hatch.kookscience.com/wiki/File:Reynolds_-_1827_-_Remarks_on_a_Review_of_Symmes%27_Theory.pdf 
• Gardner, Marshall B. (1920), A Journey to the Earth's Interior; or, Have the Poles Really Been Discovered, Aurora, Illinois: The Author, https://catalog.hathitrust.org/Record/012173485  — [revised edition, nearly 400 pages added]
• Giannini, F. Amadeo (1959), Worlds Beyond The Poles: Physical Continuity of the Universe, New York: Vantage Press, https://archive.org/details/WorldsBeyondThePolesPhysicalContinuityOfTheUniverseFinalWithAddedPages/