The Hollow Earth hypothesis proposes that the planet Earth is either entirely hollow or otherwise contains a substantial interior space. The hypothesis has been shown to be wrong by observational evidence, as well as by the modern understanding of planet formation; the scientific community has dismissed the notion since at least the late 18th century.
The concept of a hollow Earth still recurs in folklore and as the premise for subterranean fiction, a subgenre of adventure fiction. It is also featured in some present-day pseudoscientific and conspiracy theories.
Ancient history 
In ancient times, the concept of a subterranean land inside the earth appeared in mythology, folklore and legends. 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 Nordic Svartálfaheimr, the Christian Hell, and the Jewish Sheol (with details describing inner Earth in Kabalistic literature, such as the Zohar and Hesed L'Avraham). The idea of a subterranean realm is also mentioned in Tibetan Buddhism belief, according to one story there is an ancient city called Shamballa which is located inside the earth.
According to the 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 Trozien in Argolis, at Ephya in Thesprotia, at Herakleia in Pontos, and in Ermioni. In Thracians and Dacians legend it is said that there are underground chambers occupied by an ancient God called Zalmoxis. 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.
In Celtic mythology there is a legend of a cave called "Cruachan," also known as "Ireland's gate to Hell," a legendary and ancient cave from which according to legend strange creatures would emerge in ancient times and be seen on the surface of the earth. 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. There is an Irish myth which says tunnels in County Down, Northern Ireland lead to the land of the subterranean Tuatha de Danaan, a group of people who are believed to have introduced Druidism to Ireland, and then went back underground.
An ancient legend of the Angami Naga tribes of India claim that their ancestors emerged in ancient times from a subterranean land inside the earth. There are legends from the Taíno people that their ancestors emerged in ancient times from two caves in a mountain underground.
It is the belief of the natives of the Malinowski's Trobriand Islands that their ancestors had come from a subterranean land through a cavern hole called "Obukula". There is an ancient legend held in Mexican folklore that a cave in a mountain five miles south of Ojinaga, Mexico is possessed by devilish creatures who came from inside the earth.
There was an ancient myth held in the middle ages that some mountains located between Eisenach and Gotha in Germany hold a portal to the inner earth. There is an old Russian legend that says the Samoyeds, an ancient Siberian tribe, traveled to an underground cavern city to live inside the earth.
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 at the north side of the Missouri River. There is also a tale about a tunnel in the San Carlos Apache Indian Reservation in Arizona near Cedar Creek[disambiguation needed] which is said to lead inside the earth to a land inhabited by a mysterious tribe. It is also the belief of the tribes of Iroquois that their ancient ancestors emerged from a subterranean world inside the earth. The elders of the Hopi people believe that a Sipapu entrance in the Grand Canyon exists which leads to the underworld.
According to South American mythology the belief of the 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. There are also legends that say the ancestors of the Inca Empire came from underground caves which are located east of Cuzco, Peru.
17th and 18th centuries 
Edmond Halley in 1692 put forth the idea of Earth consisting of a hollow shell about 800 km (500 mi) thick, two inner concentric shells and an innermost core, about the diameters of the planets Venus, Mars, and Mercury. 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 atmosphere inside as luminous (and possibly inhabited) and speculated that escaping gas caused the Aurora Borealis.
De Camp and Ley have claimed (in their Lands Beyond) that Leonhard Euler also proposed a hollow-Earth idea, getting rid of multiple shells and postulating an interior sun 1,000 km (620 mi) across to provide light to advanced inner-Earth civilization but they provide no references; indeed, Euler did not propose a hollow-Earth, but there is a slightly related thought experiment.
De Camp and Ley also claim that Sir John Leslie expanded on Euler's idea, suggesting two central suns named Pluto and Proserpine (this was unrelated to the dwarf planet Pluto, which was discovered and named some time later). Leslie did propose a hollow Earth in his 1829 Elements of Natural Philosophy (pp. 449–453), but does not mention interior suns.
Le Clerc Milfort in 1781 led a journey with hundreds of Creek Indians to a series of caverns near the Red River above the junction of the Mississippi river, according to Milfort the original Creek Indian 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."
19th century 
In 1818, John Cleves Symmes, Jr. suggested that the Earth consisted of a hollow shell about 1,300 km (810 mi) thick, with openings about 2,300 km (1,400 mi) across at both poles with 4 inner shells each open at the poles. Symmes became the most famous of the early Hollow Earth proponents. He proposed making an expedition to the North Pole hole, thanks to efforts of one of his followers, James McBride. United States president John Quincy Adams indicated he would approve of this but he left office before this could occur. The new President of the United States, Andrew Jackson, halted the attempt. It is possible this is the source of the (untrue) legend that Jackson believed in a Flat Earth, and was consequently the only United States president to do so.
Jeremiah 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 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 about his ideas, 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, a 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.
20th century 
An early twentieth-century proponent of hollow Earth, William Reed, wrote Phantom of the Poles in 1906. He supported the idea of a hollow Earth, but without interior shells or inner sun.
The spiritualist writer Walburga, Lady Paget in her book Colloquies with an unseen friend (1907) was an early writer to mention the hollow earth theory. 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.
William Fairfield Warren, in his book, Paradise Found: The Cradle of the Human Race at the North Pole presented his belief that humanity originated on a continent in the Arctic called Hyperborea. This influenced some early hollow earth theorists. According to Marshall Gardner, both the Eskimo and Mongolian peoples had come from the interior of the earth by an entrance at the North pole.
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 patented (U.S. Patent 1,096,102). Gardner made no mention of Reed, but did criticize Symmes for his ideas. About the same time Vladimir Obruchev wrote a novel 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.
Explorer Ferdynand Ossendowski wrote a book in 1922 titled Beasts, Men and Gods. Ossendowski said he was told about a subterranean kingdom exists inside the earth. It was known to Buddhists as Agharti.
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 centre of the earth. One man entered the tunnel and never returned.
Other in the 20th century proposed that "ascended masters" of esoteric wisdom inhabit subterranean caves or a hollow Earth. Antarctica, the North Pole, Tibet, Peru, and Mount Shasta in California, USA, have all had their advocates as the locations of entrances to a subterranean realm referred to as Agartha, with some even advancing the hypothesis that aliens have their homeland in these places.
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. Michael Grumley a cryptozoologist has linked Bigfoot and other hominid cryptids to ancient tunnel systems underground.
Douglas Baker wrote in one of his books that he had an astral journey to the inner earth where he observed a subterranean civilization. Other occult writers such as Guy Ballard and Alice Bailey have written that they have had out of body experiences and met mysterious beings inside of the earth.
According to the ancient astronaut writer Peter Kolosimo a robot was seen entering a subterranean tunnel below a monastery in Mongolia, he also claimed a light was seen from underground in Azerbaijan. Kolosimo and other ancient astronaut writers such as Robert Charroux linked these activities to UFOs.
A book allegedly by a "Dr. Raymond Bernard" which appeared in 1964, The Hollow Earth, exemplifies the idea of UFOs coming from inside the earth. The book rehashes Reed and Gardner's ideas and ignores Symmes. Bernard also adds his own ideas: the Ring Nebula proves the existence of hollow worlds, as well as speculation on the fate of Atlantis and the origin of flying saucers. Bernard argued that the inhabitants of Atlantis took refuge in the Earth's interior before the city was destroyed in great calamity. It was Atlanteans who piloted the flying machines known in ancient India as vimanas and in the modern world as flying saucers. After the US bombings of Hiroshima and Nagasaki, Bernard claimed, the Atlanteans became concerned that radioactive air might flow into the world's interior, and so some emerged in their flying saucers in an act of self-defense. An article by Martin Gardner revealed that Dr. Walter Siegmeister used the pseudonym `Bernard', but not until the publishing of Walter Kafton-Minkel's Subterranean Worlds: 100,000 years of dragons, dwarves, the dead, lost races & UFOs from inside the Earth, in 1989, did the full story of Bernard/Siegmeister become well known.
The pages of the science fiction pulp magazine Amazing Stories promoted one such idea from 1945 to 1949 as "the Shaver Mystery". The magazine's editor, Ray Palmer, ran a series of stories by Richard Sharpe Shaver supposedly claimed as factual, though presented in the context of fiction. Shaver claimed 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.
Hollow earth theorists 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, Staffordshire in England, Montreal in Canada, Hangchow in China, and the Amazon Rainforest.
Fantastic stories (supposedly believed as factual within fringe circles) have also circulated that Adolf Hitler and some of his followers escaped to hollow lands within the Earth after World War II via an entrance in Antarctica. (See also Hitler's supposed adherence to concave hollow-Earth ideas, below.)
21st century 
In 2011, Horatio Valens and Paul Veneti presented a two-hour "Lazeria Map Collection" video on centuries-old maps of the Arctic region and the North Pole, making a case for a 100-mile wide canyon in the center of the physical North Pole, into which north-flowing rivers drain into a hollow Earth. The maps were collected by Harry Hubbard.
Concave hollow Earths 
Instead of saying that humans live on the outside surface of a hollow planet — sometimes called a "convex" hollow-Earth hypothesis — some have claimed humans live on the inside surface of a hollow spherical world, so that our universe itself lies in that world's interior. This has been called the "concave" hollow-Earth hypothesis.
Cyrus Teed, a doctor from upstate New York, proposed such a concave hollow Earth in 1869, calling his scheme "Cellular Cosmogony". 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 twentieth-century German writers, including Peter Bender, Johannes Lang, Karl Neupert, and Fritz Braun, 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.
In one chapter of 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, which corresponds to no point a finite distance away from Earth in the widely accepted scientific cosmology. 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 in the widely accepted scientific cosmology. Supposedly no experiment can distinguish between the two cosmologies.
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.
Purportedly verifiable hypotheses of a "concave hollow Earth" need to be distinguished from a thought experiment which defines a 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 R²/r where R is the Earth's radius.) 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.
Contrary evidence 
The picture of the structure of the earth that has been arrived at through the study of seismic waves is quite different from the hollow earth theory. The Earth's interior is made up of layers of molten rock and various elements, in the mantle and core.
Another set of scientific arguments against a hollow Earth or any hollow planet comes from gravity. Massive objects tend to clump together gravitationally, creating non-hollow spherical objects we call stars and planets. The solid sphere is the best way in which to minimize the gravitational potential energy of a physical object; having hollowness is unfavorable in the energetic sense. In addition, ordinary matter is not strong enough to support a hollow shape of planetary size against the force of gravity; a planet-sized hollow shell with the known, observed thickness of the Earth's crust, would not be able to achieve hydrostatic equilibrium with its own mass and would collapse.
Someone on the inside of a hollow Earth would not experience a significant outward pull and could not easily stand on the inner surface; rather, the theory of gravity implies that a person on the inside would be nearly weightless. This was first shown by Newton, whose shell theorem mathematically predicts a gravitational force (from the shell) of zero everywhere inside a spherically symmetric hollow shell of matter, regardless of the shell's thickness. A tiny gravitational force would arise from the fact that the Earth does not have a perfectly symmetrical spherical shape, as well as forces from other bodies such as the Moon. The centrifugal force from the Earth's rotation would pull a person (on the inner surface) outwards if the person was traveling at the same velocity as the Earth's interior and was in contact with the ground on the interior, but even the maximum centrifugal force at the equator is only 1/300 of ordinary Earth gravity.
The mass of the planet also indicates that the hollow Earth hypothesis is unfeasible. Should the Earth be largely hollow, its mass would be much lower and thus its gravity on the outer surface would be much lower than it is.
Direct observation 
The deepest hole drilled to date is the SG-3 borehole which is 12.3 km (7.6 mi) deep, part of the Soviet Kola Superdeep Borehole project; thus, visual knowledge of the Earth's structure extends that far.
In fiction 
The idea of a hollow Earth is a common element of fiction, appearing as early as Ludvig Holberg's 1741 novel Nicolai Klimii iter subterraneum (Niels Klim's Underground Travels), in which Nicolai Klim falls through a cave while spelunking and spends several years living on a smaller globe both within and the inside of the outer shell.
Other notable pre-20th century examples include Giacomo Casanova's 1788 Icosaméron, a 5-volume, 1,800-page story of a brother and sister who fall into the Earth and discover the subterranean utopia of the Mégamicres, a race of multicolored, hermaphroditic dwarfs; Symzonia: A Voyage of Discovery by a "Captain Adam Seaborn" (1820) which reflected the ideas of John Cleves Symmes, Jr.; Edgar Allan Poe's 1838 novel The Narrative of Arthur Gordon Pym of Nantucket; Jules Verne's 1864 novel A Journey to the Center of the Earth, which described a prehistoric subterranean world; and George Sand's 1884 novel Laura, Voyage dans le Cristal where unseen and giant crystals could be found in the interior of the Earth.
In W.H. Hudson's 1887 romance, A Crystal Age, the protagonist falls down a hill into a Utopian, asexual, pastoral paradise; since he falls into this world, it is sometimes classified as a hollow earth story; although, the hero himself, thinks he may have traveled forward in time, by millenia.
The idea was used by Edgar Rice Burroughs, the creator of Tarzan, in the seven-novel "Pellucidar" series, beginning with At the Earth's Core (1914). Using a mechanical drill, his heroes discover a prehistoric world, called Pellucidar, 500 miles below the surface, that is lit by an inner sun. The 1915 novel Plutonia by Vladimir Obruchev uses the concept of the hollow Earth to take the reader through various geological epochs.
In more recent decades, the idea has become a staple of the science fiction and adventure genres, appearing in print, in film, on television, in comics, in role-playing games, and in many animated works.
See also 
- Hollow Earth in the Puranas Online
- The Way to Shambhala, Edwin Bernbaum, Anchor Books; 1st edition, 1980 ISBN 0-385-12794-4
- William Sherwood Fox, Greek and Roman Mythology; p. 143
- Mircea Eliade, Zalmoxis, the vanishing God: comparative studies in the religions and folklore of Dacia and Eastern Europe, 1959, pp. 24–30
- Myth: its meaning and functions in ancient and other cultures, G. S. Kirk, 1970, p. 136
- John A MacCulloch, Celtic Mythology, Rowman & Littlefield Pub Inc, 1932, pp. 125–126
- T. Write, Saint Patrick's Purgatory: A medieval Pilgrimage in Ireland, 1918, p. 107
- Harold Bayley, Archaic England: An Essay in Deciphering Prehistory from Megalithic Monuments, 1919 Online Edition: Link
- Angami NagaBrown, Account of Munnipore, 1968., p. 113
- 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."
- Philip Freund, Myths of Creation; 1965, pp. 131–132
- George, Wally – Pilgrimage To The Devil., Article in Fate magazine, Aug. 1957, pp. 38–52
- Clark B Firestone and Ruth Hambidge, The Coasts of Ilusion, Harper & Bros; First Edition, 1924
- Martha Warren Beckwith, Mandan-Hidatsa myths and ceremonies, G. E. Stechert, 1937, p. 10
- Grenville Goodwin, Myths and Tales of the White Mountain Apache, 1939, p. 20 (Kessinger Publishing have reprinted the book in 2011)
- William Martin Beauchamp, Iroquois folk lore: gathered from the Six Nations of New York, I. J. Friedman, 1965, pp. 152–153
- Pages from Hopi history, Harry Clebourne James, University of Arizona Press, 1974, Chapter 6
- Arizona and the West, Volume 17, University of Arizona Press., 1975, p. 179
- Harold Osbourne, South American Mythology. New York: Peter Bedrick Books, 1986, p. 42 and p. 119
- 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
- Halley, Edmond, An Account of the Late Surprizing 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 406–428
- Euler and the Hollow Earth: Fact or Fiction?
- Migration Legend of the Creek Indians, Volumes 1–2, Albert S. Gatschet, Ams Pr Inc, 1969
- 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
- Paget Walburga, Colloquies with an unseen friend, William Rider & Son., London., 1909., p. 36
- A Journey to the Earth's Interior, Marshall Gardner, Mokelumne Hill Pr, 1974 Edition, ISBN 0-7873-1192-8
- Ferdynand Ossendowski (1922). Beasts, Men and Gods. New York: E. P. Dutton & Company.
- George & Helen Papashvily, – Anything Can Happen., Harper & Bros., New York, NY., 1940
- Cave of the Ancients, Lobsang Rampa, Random House, 1993
- There are Giants in the Earth, Michael Grumley, Panther Books, 1976, pp. 42–47
- Douglas Baker, Practical Techniques of Astral Projection, The Aquarian Press, Wellingboro, Northamptonshire, England, 1977, p. 52
- 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
- Reece, Gregory L. (August 21, 2007). UFO Religion: Inside Flying Saucer Cults and Culture. I. B. Tauris. p. 17. ISBN 1-84511-451-5.
- 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
- David Hatcher Childress Lost Continents and the Hollow Earth Adventures Unlimited Press, 1998 ISBN 978-0932813633
- Alien races and Fantastic Civilizations., Serge Hutin, Berkeley Medallion Books, 1975, pp. 109–132 – In the Bowels of the Earth: Refers to the mysterious catacombs beneath Paris, and other underground mysteries which lead inside the earth.
- The Under-People, Eric Norman, Award Books, 1969
- Inner Earth People And Outer Space People, William L. Blessing, Inner Light Publications, 2008 Edition ISBN 1-60611-036-5
- Chinese ghouls and goblins, G Willoughby-Meade, Stokes co, 1929
- Mysteries of Ancient South America, Harold T. Wilkins, Citadel Press., New York, 1956
- Lazeria Map Collection: Is the Earth Hollow? on YouTube
- Kuiper, Gerard. P. (June, 1946). "German Astronomy during the War". Popular Astronomy 54: 263–286. Bibcode:1946PA.....54..263K. See pages 277 – 278.
- Yenne, William (2003). "Adolf Hitler and the Concave Earth Cult". Secret Weapons of World War II: The Techno-Military Breakthroughs That Changed History. New York: Berkley Books. pp. 271–272. ISBN 0425189929.
- Abdelkader, M. (1983). "A Geocosmos: Mapping Outer Space Into a Hollow Earth". Speculations in Science & Technology (6): 81–89.
- Notices of the American Mathematical Society, (Oct. 1981 and Feb. 1982).
- On the Wild Side (1992), Martin Gardner, p.18-9
- On the Wild Side, 1992, Martin Gardner.\
- Press, Frank; Siever, Raymond; Grotzinger, John; Jordan, Tom (2003). Understanding Earth (4 ed.). New York, New York: W. H. Freeman. pp. 484–487. ISBN 0-7167-9617-1.
- USGS - Earth's interior
- Eagleson, Mary (1994). Concise Encyclopedia Chemistry. Walter de Gruyter, p799. ISBN 3-11-011451-8
- At the Earth's Core, by Edgar Rice Burroughs
- Seaborn, Captain Adam. Symzonia; Voyage of Discovery. J. Seymour, 1820.
- Lewis, David. The Incredible Cities of Inner Earth. Science Research Publishing House, 1979.
- Kafton-Minkel, Walter. Subterranean Worlds. Loompanics Unlimited, 1989.
- Standish, David. Hollow Earth. Da Capo Press, 2006.
- The Hollow Earth – from The UnMuseum
- Hollow Orbs
- The North Pole Inner Earth Expedition official website
- Skeptic Dictionary: Hollow Earth
- What Curiosity in the Structure: The Hollow Earth in Science.
- Some references
- About the Jan Lamprecht's book
- Interview of Jan Lamprecht about hollow earth and science
- (fr) Many references about the claimed Admiral Byrd's journey to inner earth and some facts about north pole