Question: Subterranean fiction and the Hollow Earth theory are based on what kind of concept?
Context: Invisible Eagle - The History Of Nazi Occultism is a book written by Alan Baker.  It was published in 2000 by Virgin Books.  Among other subjects the book deals with themes of Hollow Earth theory, Welteislehre, foo fighters, Vril Society and similar speculative theories that were associated with the Third Reich. The Hollow Earth is a pseudoscience concept proposing 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, and a subgenre of adventure fiction ("Journey to the Center of the Earth", "At the Earth's Core").  It is also featured in some present-day pseudoscientific and conspiracy theories. Cyrus Reed Teed (October 18, 1839 – December 22, 1908) was a U.S. eclectic physician and alchemist turned religious leader and messiah.  In 1869, claiming divine inspiration, Dr. Teed took on the name Koresh and proposed a new set of scientific and religious ideas he called Koreshanity, including a unique Hollow Earth theory that posits the Earth and sky exist inside the inner surface of a sphere. Subterranean fiction is a subgenre of adventure fiction or science fiction which focuses on underground settings, sometimes at the center of the Earth or otherwise deep below the surface.  The genre is based on and has in turn influenced the Hollow Earth theory. Travelling to the Earth's center is a popular theme in science fiction.  Some subterranean fiction involves traveling to the Earth's center and finding either a Hollow Earth or Earth's molten core. Before the concept of plate tectonics, global cooling was a geophysical theory by James Dwight Dana, also referred to as the contracting earth theory.  It suggested that the Earth had been in a molten state, and features such as mountains formed as it cooled and shrank.  As the interior of the Earth cooled and shrank, the rigid crust would have to shrink and crumple.  The crumpling could produce features such as mountain ranges. Warren Billy Smith, (Iowa, 1931–2003) was an American author who wrote books on cryptozoology and the hollow earth theory. John Cleves Symmes Jr. (November 5, 1780 – May 28, 1829) was an American Army officer, trader, and lecturer.  Symmes is best known for his 1818 variant of the Hollow Earth Theory, which introduced the concept of openings to the inner world at the poles. We Rule the Night is the fourth studio album by Swedish metalcore band Sonic Syndicate.  The album was released on August 27, 2010 through Nuclear Blast and was produced by Toby Wright.  "We Rule the Night" is the first full-length release by the band to feature the vocals of Nathan James Biggs (former Hollow Earth Theory singer) who replaced former singer Roland Johansson who left the band in 2009.  However, it is also the last album to feature founding members Richard and Roger Sjunnesson, who both left the band in October 2010 and May 2012 respectively.  The band toured Europe to support the album with the "We Rule the World Tour", starting on September 15, 2010 with Christoffer Andersson standing in for Richard. Hollow Earth Expedition is a pulp fiction role-playing game set in the fictitious Hollow Earth (see Hollow Earth theory), published by Exile Game Studio.  The game has been nominated for several Origins and ENnie awards since its release in 2006.  The main rule book is "Hollow Earth Expedition".
Answer: a pseudoscience