Editing Hollow Earth (section)

== History ==

===Ancient times===
In ancient times, the concept of a subterranean land inside the Earth appeared in [[mythology]], [[folklore]] and [[legend]]s. The idea of subterranean realms seemed arguable, and became intertwined with the concept of "places" of origin or afterlife, such as the [[Greek underworld]], the [[Norse mythology|Nordic]] [[Svartálfaheimr]], the Christian [[Hell]], and the Jewish [[Sheol]] (with details describing inner Earth in [[Kabbalah|Kabalistic]] literature, such as the [[Zohar]] and [[Avraham Azulai|Hesed L'Avraham]]). The idea of a subterranean realm is also mentioned in [[Tibetan Buddhist]] belief.<ref>''Hollow Earth in the Puranas'' [http://holloworbs.com/HE_Puranas_Article.htm Online]</ref><ref name="ReferenceA">''The Way to Shambhala'', Edwin Bernbaum, Anchor Books; 1st edition, 1980 {{ISBN|0-385-12794-4}}</ref> According to one story from Tibetan Buddhist tradition, there is an ancient city called [[Shamballa]] which is located inside the Earth.<ref name="ReferenceA" />

According to the [[Ancient Greece|Ancient Greeks]], there were [[caverns]] under the surface which were entrances leading to the [[underworld]], some of which were the caverns at [[Tainaron]] in [[Lakonia]], at [[Troezen]] in [[Argolis]], at Ephya in [[Thesprotia]], at Herakleia in [[Pontus (region)|Pontos]], and in [[Ermioni]].<ref>{{citation | volume = 1 | year = 1916 | first = William | last = Sherwood Fox | author-link = William Sherwood Fox | title = Greek and Roman | url = https://archive.org/details/mythologyofallra11gray_0 | page= 143| publisher = Boston, Marshall Jones Company }}</ref> In [[Thracians|Thracian]] and [[Dacians|Dacian]] legends, it is said that there are [[Environmental chamber|caverns]] occupied by an ancient god called [[Zalmoxis]].<ref>Mircea Eliade, ''Zalmoxis, the vanishing God: comparative studies in the religions and folklore of Dacia and Eastern Europe'', 1959, pp. 24–30</ref> In [[Mesopotamian religion]] there is a story of a man who, after traveling through the darkness of a tunnel in the mountain of "Mashu", entered a subterranean garden.<ref>''Myth: its meaning and functions in ancient and other cultures'', G. S. Kirk, 1970, p. 136</ref>

[[File:Station Island.jpg|thumb|Chapel, bell tower and penitential beds on [[Station Island]]. The bell tower stands on a mound that is the site of a cave which, according to various myths, is an entrance to a place of [[purgatory]] inside the Earth. The cave has been closed since October 25, 1632.]]

In [[Celtic mythology]] there is a legend of a cave called "[[Cruachan, Ireland|Cruachan]]", also known as "Ireland's gate to Hell", a mythical and ancient cave from which strange creatures would emerge and be seen on the surface of the Earth.<ref>John A MacCulloch, Celtic Mythology, Rowman & Littlefield Pub Inc, 1932, pp. 125–26</ref> There are also stories of medieval knights and saints who went on pilgrimages to a cave located in [[Station Island]], County Donegal in Ireland, where they made journeys inside the Earth into a place of [[purgatory]].<ref>T. Write, ''Saint Patrick's Purgatory : A medieval Pilgrimage in Ireland'', 1918, p. 107</ref> In [[County Down]], Northern Ireland there is a myth which says tunnels lead to the land of the subterranean [[Tuatha Dé Danann]], a group of people who are believed to have introduced [[Druidism]] to Ireland, and then went back underground.<ref>Harold Bayley, ''Archaic England: An Essay in Deciphering Prehistory from Megalithic Monuments'', 1919 Online Edition: [https://books.google.com/books?id=hHD5DFeFTZ0C&dq=Bayley,+Harold+-+ARCHAIC+ENGLAND+county+down&pg=PA766 Link]</ref>

In Hindu mythology, the underworld is referred to as [[Patala]]. In the Bengali version of the Hindu epic [[Ramayana]], it has been depicted how [[Rama]] and [[Lakshmana]] were taken by the king of the underworld [[Ahiravan]], brother of the demon king [[Ravana]]. Later on they were rescued by [[Hanuman]]. The [[Angami Naga]] tribes of [[India]] claim that their ancestors emerged in ancient times from a subterranean land inside the Earth.<ref>Angami NagaBrown, ''Account of Munnipore'', 1968., p. 113</ref> The [[Taíno people|Taino]] from Cuba believe their ancestors emerged in ancient times from two caves in a mountain underground.<ref>Ellen Russell Emerson, ''Indian Myths'', 1965 "It is to the Cubans we are indebted for the following version of man's origin: It was from the depths of a deep cavern in the earth that mankind issued."</ref>

Natives of the [[Trobriand Islands]] believe that their ancestors had come from a subterranean land through a cavern hole called "Obukula".<ref>Philip Freund, ''Myths of Creation''; 1965, pp. 131–32</ref> Mexican folklore also tells of a cave in a mountain five miles south of [[Ojinaga]], and that Mexico is possessed by devilish creatures who came from inside the Earth.<ref>George, Wally&nbsp;–"Pilgrimage To The Devil"., Article in ''Fate magazine'', Aug. 1957, pp. 38–52</ref>

In the [[Middle Ages]], an ancient German myth held that some mountains located between [[Eisenach]] and [[Gotha]] hold a portal to the inner Earth. A Russian legend says the [[Samoyedic peoples|Samoyeds]], an ancient [[Siberian]] [[tribe]], traveled to a cavern city to live inside the Earth.<ref>Clark B Firestone and Ruth Hambidge, ''The Coasts of Ilusion'', Harper & Bros; First Edition, 1924</ref> The Italian writer [[Dante]] describes a hollow earth in his well-known 14th-century work ''[[Inferno (Dante)|Inferno]]'', in which the fall of Lucifer from heaven caused an enormous funnel to appear in previously solid and spherical earth, as well as an enormous mountain opposite it, "Purgatory".

In [[Native American mythology]], it is said that the ancestors of the [[Mandan people]] in ancient times emerged from a subterranean land through a cave on the north side of the [[Missouri River]].<ref>Martha Warren Beckwith, ''Mandan-Hidatsa myths and ceremonies'', G. E. Stechert, 1937, p. 10</ref> There is also a tale about a tunnel in the [[San Carlos Apache Indian Reservation]] in [[Arizona]] near [[Help:Disambiguation|Cedar Creek]] which is said to lead inside the Earth to a land inhabited by a mysterious tribe.<ref>Grenville Goodwin, ''Myths and Tales of the White Mountain Apache'', 1939, p. 20 (Kessinger Publishing have reprinted the book in 2011)</ref> It is also the belief of the [[tribe]]s of the [[Iroquois]] that their ancient ancestors emerged from a subterranean world inside the Earth.<ref>William Martin Beauchamp, ''Iroquois folk lore: gathered from the Six Nations of New York'', I. J. Friedman, 1965, pp. 152–153</ref> The elders of the [[Hopi people]] believe that a [[Sipapu]] entrance in the [[Grand Canyon]] exists which leads to the [[underworld]].<ref>''Pages from Hopi history'', Harry Clebourne James, University of Arizona Press, 1974, Chapter 6</ref><ref>Arizona and the West, Volume 17, University of Arizona Press., 1975, p. 179</ref>

[[Brazilian Indians]], who live alongside the [[Parima River]] in Brazil, claim that their forefathers emerged in ancient times from an underground land, and that many of their ancestors still remained inside the Earth. Ancestors of the [[Inca Empire|Inca]] supposedly came from caves which are located east of [[Cuzco]], Peru.<ref>Harold Osbourne, ''South American Mythology''. New York: Peter Bedrick Books, 1986, pp. 42, 119</ref>

=== 16th to 18th centuries ===
[[File:Hollow Earth.svg|thumb|Diagram of [[Edmond Halley]]'s hypothesis]]

The notion was proposed by [[Athanasius Kircher]]'s non-fiction ''[[Mundus Subterraneus (book)|Mundus Subterraneus]]'' (1665), which speculated that there is an "intricate system of cavities and a channel of water connecting the poles".<ref name=":1">{{Cite book|last=Stableford|first=Brian M.|url=https://books.google.com/books?id=uefwmdROKTAC&pg=PA138|title=Science Fact and Science Fiction: An Encyclopedia|date=2006|publisher=Taylor & Francis|isbn=978-0-415-97460-8|pages=137–139|language=en}}</ref>{{Rp|137}}

[[Edmond Halley]] in 1692<ref name="Halley">Halley, Edmond, "An Account of the cause of the Change of the Variation of the Magnetic Needle; with an Hypothesis of the Structure of the Internal Parts of the Earth", ''Philosophical Transactions of Royal Society of London'', No. 195, 1692, pp. 563–578</ref> started with Newton's (erroneous) estimate that the density of the Moon was 9/5 the density of Earth.<ref name="Kollerstrom-1992">{{Cite journal |last=Kollerstrom |first=N. |date=1992-08-01 |title=The Hollow World of Edmond Halley |url=https://journals.sagepub.com/doi/10.1177/002182869202300304 |journal=Journal for the History of Astronomy |language=en |volume=23 |issue=3 |pages=185–192 |doi=10.1177/002182869202300304 |issn=0021-8286}}</ref> Rather than assume a dense Moon
Halley conjectured that the Earth might consist of a hollow shell about {{convert|800|km|mi|abbr=on}} thick, two inner concentric shells and an innermost core. Atmospheres separate these shells, and each shell has its own magnetic poles. The spheres rotate at different speeds. Halley proposed this scheme in order to explain anomalous compass readings. He envisaged the [[Celestial body atmosphere|atmosphere]] inside as [[luminosity|luminous]] (and possibly inhabited) and speculated that escaping gas caused the [[Aurora Borealis]].<ref name="halley2">Halley, Edmond, "An Account of the Late Surprizing [sic!] Appearance of the Lights Seen in the Air, on the Sixth of March Last; With an Attempt to Explain the Principal Phaenomena thereof", ''Philosophical Transactions of Royal Society of London'', No. 347 (1716), pp. 406–28</ref>

[[Le Clerc Milfort]] in 1781 led a journey with hundreds of [[Muscogee|Muscogee Peoples]] to a series of [[caverns]] near the [[Red River of the South|Red River]] above the junction of the [[Mississippi River]]. According to Milfort the original Muscogee Peoples' ancestors are believed to have emerged out to the surface of the Earth in ancient times from the caverns. Milfort also claimed the caverns they saw "could easily contain 15,000 – 20,000 families".<ref>''Migration Legend of the Creek Indians'', Volumes 1–2, Albert S. Gatschet, Ams Pr Inc, 1969</ref><ref>''The Franco-American Review'', Volumes 1–2, the Yale University Press, 1938, p. 111. Also see ''The Venus Calendar Observatory at Aztec New Mexico'', Allan Macgillivray III, 2010, p. 25</ref>

It is often claimed that mathematician [[Leonhard Euler]] proposed a single-shell hollow Earth with a small sun (1,000 kilometres across) at the center, providing light and warmth for an inner-Earth civilization, but that is not true. Instead, he did a thought experiment of an object dropped into a hole drilled through the center, unrelated to a hollow Earth.<ref>{{cite web |last1=Sandifer |first1=Edward |title=Euler and the Hollow Earth: Fact or Fiction? |url=http://eulerarchive.maa.org/hedi/HEDI-2007-04.pdf |website=The Mathematical Association of America |access-date=12 September 2021 |date=April 2007|doi=|pages=209–214}}</ref>

=== 19th century ===
In 1818, [[John Cleves Symmes, Jr.]] suggested that the Earth consisted of a hollow shell about {{convert|1300|km|mi|abbr=on}} thick, with openings about {{convert|2300|km|mi|abbr=on}} across at both [[geographical pole|poles]] with 4 inner shells each open at the poles. Symmes became the most famous of the early Hollow Earth proponents, and [[Hamilton, Ohio]] even has a monument to him and his ideas.<ref name="atlasobscura">{{cite web|url=http://www.atlasobscura.com/places/hollow-earth-monument |title=Hollow Earth Monument &#124; Atlas Obscura: John Symmes Hollow Earth monument |publisher=atlasobscura.com|access-date=1 November 2015}}</ref> He proposed making an expedition to the [[North Pole]] hole,<ref>{{cite magazine |last1=Simon |first1=Matt |title=Fantastically Wrong: The Real-Life Journey to the Center of the Earth That Almost Was |magazine=Wired |url=https://www.wired.com/2014/10/fantastically-wrong-journey-to-the-center-of-the-earth/ |access-date=18 September 2019}}</ref> thanks to efforts of one of his followers, [[James McBride (pioneer)|James McBride]].

[[J. N. Reynolds]] also delivered lectures on the "Hollow Earth" and argued for an expedition. Reynolds went on an expedition to Antarctica himself but missed joining the [[Wilkes Expedition|Great U.S. Exploring Expedition]] of 1838–1842, even though that venture was a result of his agitation.

Though Symmes himself never wrote a book on the subject, several authors published works discussing his ideas. McBride wrote ''Symmes' Theory of Concentric Spheres'' in 1826. It appears that Reynolds has an article that appeared as a separate booklet in 1827: ''Remarks of Symmes' Theory Which Appeared in the American Quarterly Review.'' In 1868, professor W.F. Lyons published ''The Hollow Globe'' which put forth a Symmes-like Hollow Earth hypothesis, but failed to mention Symmes himself. Symmes's son Americus then published ''The Symmes' Theory of Concentric Spheres'' in 1878 to set the record straight.

[[Sir John Leslie]] proposed a hollow Earth in his 1829 ''Elements of Natural Philosophy'' (pp.&nbsp;449–53).

[[William Fairfield Warren]], in his book ''Paradise Found – The Cradle of the Human Race at the North Pole'' (1885), presented his belief that humanity originated on a continent in the Arctic called [[Hyperborea]]. This influenced some early Hollow Earth proponents. According to Marshall Gardner, both the [[Eskimo]] and [[Mongolian peoples]] had come from the interior of the Earth through an entrance at the [[North pole|North Pole]].<ref>''A Journey to the Earth's Interior'', Marshall Gardner, Mokelumne Hill Pr, 1974 Edition, {{ISBN|0-7873-1192-8}}</ref>

=== 20th century ===
''[[NEQUA or The Problem of the Ages]]'', first serialized in a newspaper printed in Topeka, Kansas in 1900 and considered an early [[feminist utopia]]n novel, mentions John Cleves Symmes' theory to explain its setting in a hollow Earth.

An early 20th-century proponent of hollow Earth, [[William Reed (author)|William Reed]], wrote ''[[Phantom of the Poles]]'' in 1906. He supported the idea of a hollow Earth, but without interior shells or the inner sun.

The [[Spiritualism (movement)|spiritualist]] writer [[Walburga, Lady Paget]] in her book ''[[Colloquies with an unseen friend]]'' (1907) was an early writer to mention the hollow Earth hypothesis. She claimed that cities exist beneath a desert, which is where the people of [[Atlantis]] moved. She said an entrance to the subterranean kingdom will be discovered in the 21st century.<ref>Paget Walburga, ''Colloquies with an unseen friend'', William Rider & Son., London, 1909, p. 36</ref>

Marshall Gardner wrote ''[[A Journey to the Earth's Interior]]'' in 1913 and published an expanded edition in 1920. He placed an interior sun in the Earth and built a working model of the Hollow Earth which he patented ({{US patent|1096102}}). Gardner made no mention of Reed, but did criticize Symmes for his ideas. Around the same time, [[Vladimir Obruchev]] wrote a novel titled ''[[Vladimir Obruchev|Plutonia]]'', in which the Hollow Earth possessed an inner Sun and was inhabited by prehistoric species. The interior was connected with the surface by an opening in the [[Arctic]].

The explorer [[Ferdynand Antoni Ossendowski|Ferdynand Ossendowski]] wrote a book in 1922 titled ''Beasts, Men and Gods''. Ossendowski said he was told about a subterranean kingdom that exists inside the Earth. It was known to [[Buddhists]] as [[Agartha|Agharti]].<ref>Ferdynand Ossendowski (1922). ''Beasts, Men and Gods''. New York: E. P. Dutton & Company.</ref>

[[George Papashvily]] in his ''Anything Can Happen'' (1940) claimed the discovery in the [[Caucasus Mountains]] of a cavern containing human skeletons "with heads as big as bushel baskets" and an ancient tunnel leading to the center of the Earth. One man entered the tunnel and never returned.<ref>George & Helen Papashvily,&nbsp;– ''Anything Can Happen''., Harper & Bros., New York, NY., 1940</ref>

Novelist [[Lobsang Rampa]] in his book ''[[The Cave of the Ancients]]'' said an underground chamber system exists beneath the [[Himalayas]] of [[Tibet]], filled with ancient machinery, records and treasure.<ref>''Cave of the Ancients'', Lobsang Rampa, [[Random House]], 1993</ref> [[Michael Grumley]], a [[cryptozoologist]], has linked [[Bigfoot]] and other [[hominid]] [[cryptids]] to ancient tunnel systems underground.<ref>''There are Giants in the Earth'', Michael Grumley, Panther Books, 1976, pp. 42–47</ref>

According to the [[ancient astronaut]] writer [[Peter Kolosimo]] a robot was seen entering a tunnel below a [[monastery]] in Mongolia. Kolosimo also claimed a light was seen from underground in Azerbaijan.<ref>[[Peter Kolosimo]], ''Not of this World'', Sphere Books 1974 {{ISBN|0-7221-5309-0}} also see Peter Kolosimo, Timeless Earth, Citadel Pr, 1988 Edition {{ISBN|0-8065-1070-6}}</ref> Kolosimo and other ancient astronaut writers such as [[Robert Charroux]] linked these activities to [[Unidentified flying object|UFOs]].

A book by "Dr. [[Raymond W. Bernard|Raymond Bernard]]" which appeared in 1964, ''The Hollow Earth'', exemplifies the idea of UFOs coming from inside the Earth, and adds the idea that the [[Ring Nebula]] proves the existence of hollow worlds, as well as speculation on the fate of [[Atlantis]] and the origin of flying saucers.<ref>{{cite book|last=Reece|first=Gregory L.|title=UFO Religion: Inside Flying Saucer Cults and Culture|url=https://archive.org/details/uforeligioninsid00reec|url-access=limited|publisher=I. B. Tauris|year=2007|page=[https://archive.org/details/uforeligioninsid00reec/page/n27 17]|isbn=978-1-84511-451-0}}</ref> An article by [[Martin Gardner]] revealed that Walter Siegmeister used the pseudonym "Bernard", but not until the 1989 publishing of Walter Kafton-Minkel's ''Subterranean Worlds: 100,000 Years of Dragons, Dwarfs, the Dead, Lost Races & UFOs from Inside the Earth'' did the full story of Bernard/Siegmeister become well-known.<ref>Walter Kafton-Minkel ''Subterranean Worlds: 100,000 Years of Dragons, Dwarfs, the Dead, Lost Races and Ufos from Inside the Earth'' [[Loompanics Unlimited]], 1989 {{ISBN|978-1559500159}}</ref>

The science fiction [[pulp magazine]] ''[[Amazing Stories]]'' promoted one such idea from 1945 to 1949 as "The Shaver Mystery". The magazine's editor, [[Raymond A. Palmer|Ray Palmer]], ran a series of stories by [[Richard Sharpe Shaver]], claiming that a superior pre-historic race had built a [[honeycomb]] of caves in the Earth, and that their degenerate descendants, known as "Dero", live there still, using the fantastic machines abandoned by the ancient races to torment those of us living on the surface. As one characteristic of this torment, Shaver described "voices" that purportedly came from no explainable source. Thousands of readers wrote to affirm that they, too, had heard the fiendish voices from inside the Earth. The writer [[David Hatcher Childress]] authored ''Lost Continents and the Hollow Earth'' (1998) in which he reprinted the stories of Palmer and defended the Hollow Earth idea based on alleged tunnel systems beneath South America and Central Asia.<ref>[[David Hatcher Childress]] ''Lost Continents and the Hollow Earth ''Adventures Unlimited Press, 1998 {{ISBN|978-0932813633}}</ref>

Hollow Earth proponents have claimed a number of different locations for the entrances which lead inside the Earth. Other than the North and South poles, entrances in locations which have been cited include: Paris in France,<ref>''Alien races and Fantastic Civilizations'', Serge Hutin, Berkeley Medallion Books, 1975, pp. 109–132&nbsp;– In the Bowels of the Earth: Refers to the mysterious catacombs beneath Paris, and other underground mysteries which lead inside the Earth.</ref> [[Staffordshire]] in England,<ref>''The Under-People'', Eric Norman, Award Books, 1969</ref> [[Montreal]] in Canada,<ref>''Inner Earth People And Outer Space People'', William L. Blessing, Inner Light Publications, 2008 Edition {{ISBN|1-60611-036-5}}</ref> [[Hangzhou]] in China,<ref>''Chinese ghouls and goblins'', G Willoughby-Meade, Stokes co, 1929</ref> and the [[Amazon rainforest]].<ref>''Mysteries of Ancient South America'', Harold T. Wilkins, Citadel Press.', New York, 1956</ref>

=== Variations ===

In "[[A Culture of Conspiracy]]", Political scientist [[Michael Barkun]] draws a distinction between the terms ''hollow earth'' and ''inner earth'', to differentiate materials that conceive the majority of the interior of the planet to be hollow, from those that view it as solid but [[honeycomb]]ed with interconnected spaces.<ref>{{cite book |last1=Michael Barkun |title=[[A Culture of Conspiracy]] |date=2003 |publisher=[[University of California Press]] |isbn=0-520-23805-2 |page=207 |edition=1st |author1-link=Michael Barkun }}</ref><ref>{{cite book |last1=Kafton-Minkel |first1=Walter |title=Subterranean Worlds: 100,000 Years of Dragons, Dwarfs, the Dead, Lost Races & UFOs from Inside the Earth |date=1989 |publisher=[[Loompanics]] |location=[[Port Townsend]] |isbn=9781559500159 |pages=44–55 |url=https://shwetathanki.files.wordpress.com/2020/05/subterranean-worlds-walter-kafton-minkel-100000-years-of-dragons-dwarfs-the-dead-lost-races-ufos-from-inside-the-earth.pdf}}</ref><ref>{{cite book |last1=Radner, Radne |first1=Daisie, Michael |title=Science and Unreason |date=1982 |publisher=[[Cengage Group|Wadsworth]] |location=[[Belmont, California]] |isbn=9780534011536 |pages=48–50}}</ref>

==== Concave Hollow Earths ====
[[File:Concave hollow Earths.svg|thumb|200px|An example of a concave hollow Earth. Humans live on the interior, with the universe in the center.]]

Instead of saying that humans live on the exterior surface of a hollow planet, sometimes called a "convex" Hollow Earth hypothesis, it is hypothesized humans live on the ''interior'' surface. This has been called the "concave" Hollow Earth hypothesis or skycentrism.

[[Cyrus Teed]], a doctor from upstate New York, proposed such a concave Hollow Earth in 1869, calling his scheme "Cellular Cosmogony".<ref name="wired">{{cite magazine|url=https://www.wired.com/2014/07/fantastically-wrong-hollow-earth/ |title=Fantastically Wrong: The Legendary Scientist Who Swore Our Planet Is Hollow &#124; WIRED |magazine=Wired |publisher=wired.com|access-date=1 November 2015|date=2014-07-02 }}</ref> Teed founded a group called the [[Koreshan Unity]] based on this notion, which he called [[Koreshanity]]. The main colony survives as a preserved Florida state historic site, at [[Estero, Florida]], but all of Teed's followers have now died. Teed's followers claimed to have experimentally verified the concavity of the Earth's curvature, through surveys of the Florida coastline making use of "rectilineator" equipment.

Several 20th-century German writers, including [[Peter Bender]], Johannes Lang, Karl Neupert, and Fritz Braut, published works advocating the Hollow Earth hypothesis, or ''Hohlweltlehre''. It has even been reported, although apparently without historical documentation, that [[Adolf Hitler]] was influenced by concave Hollow Earth ideas and sent an expedition in an unsuccessful attempt to spy on the British fleet by pointing infrared cameras up at the sky.<ref>{{cite journal | last = Kuiper | first = Gerard. P. | author-link = Gerard Kuiper
| title = German Astronomy during the War | journal = Popular Astronomy
| volume = 54 | pages = 263–286
| date = June 1946
| bibcode = 1946PA.....54..263K
}} See pp. 277–78.</ref><ref>{{cite book |first=William |last=Yenne |chapter=Adolf Hitler and the Concave Earth Cult |title=Secret Weapons of World War II: The Techno-Military Breakthroughs That Changed History |location=New York |publisher=Berkley Books |year=2003 |pages=271–272 |isbn=978-0-425-18992-4 }}</ref>

The [[Egypt]]ian mathematician [[Mostafa A. Abdelkader|Mostafa Abdelkader]] wrote several scholarly papers working out a detailed mapping of the Concave Earth model.<ref>{{cite journal|first=M.|last=Abdelkader|title=A Geocosmos: Mapping Outer Space Into a Hollow Earth|issue=6|journal=Speculations in Science & Technology|pages=81–89|year=1983}}</ref><ref>''Notices of the American Mathematical Society,'' (Oct. 1981 and Feb. 1982).</ref> In his book ''On the Wild Side'' (1992), [[Martin Gardner]] discusses the Hollow Earth model articulated by Abdelkader. According to Gardner, this hypothesis posits that light rays travel in circular paths, and slow as they approach the center of the spherical star-filled cavern. No energy can reach the center of the cavern. A drill, Gardner says, would lengthen as it traveled away from the cavern and eventually pass through the "point at infinity" corresponding to the center of the Earth. Gardner notes that "most mathematicians believe that an inside-out universe, with properly adjusted physical laws, is empirically irrefutable". Gardner rejects the concave Hollow Earth hypothesis on the basis of [[Occam's razor]].<ref>''On the Wild Side'' (1992), Martin Gardner, pp. 18–19</ref>

Purportedly verifiable hypotheses of a Concave Hollow Earth need to be distinguished from a thought experiment which defines a [[Coordinates (elementary mathematics)|coordinate]] transformation such that the interior of the Earth becomes "exterior" and the exterior becomes "interior". (For example, in spherical coordinates, let radius ''r'' go to ''R''<sup>2</sup>/''r'' where ''R'' is the Earth's radius; see [[inversive geometry]].) The transformation entails corresponding changes to the forms of physical laws. This is not a hypothesis but an illustration of the fact that any description of the physical world can be equivalently expressed in more than one way.<ref name="gardnertransformation">On the Wild Side, 1992, [[Martin Gardner]].</ref>