Concave (Cellular) Earth Theory

Overview

Most people who have heard of fringe cosmological theories are familiar with Flat Earth — the belief that the Earth is a disc rather than a sphere. But Flat Earth has a less famous, arguably stranger cousin: Concave Earth theory, also known as Cellular Cosmogony or (in German) Hohlweltlehre. It is, in a sense, Flat Earth’s photographic negative.

Concave Earth theory holds that we live on the inside surface of a hollow sphere. The sky you see above you is not the outer expanse of a vast universe — it is the interior of the Earth itself. The Sun, Moon, planets, and stars are all suspended within this sphere, tiny objects floating in a central void. The universe is not infinite; it fits inside the Earth. The ground beneath your feet curves up, not down. If you could see far enough, you would see the other side of the world arching overhead.

It sounds like something a science fiction author dreamed up after too much absinthe. But Concave Earth theory has a surprisingly long pedigree, including a failed American utopian community, a bizarre experiment on the Florida Gulf Coast, and — most unsettlingly — alleged interest from the Nazi regime. And while it has far fewer adherents than Flat Earth, the theory persists in small online communities, nourished by the same distrust of mainstream science and the same fascination with heterodox cosmology.

Origins & History

Cyrus Teed and the Illumination

The most important figure in Concave Earth history is Cyrus Reed Teed, an American physician and alchemist from upstate New York. In 1869, while conducting experiments in his home laboratory (what exactly he was doing remains unclear — electrochemistry, possibly), Teed experienced what he described as a divine illumination. A beautiful woman appeared to him and revealed the true nature of the universe: the Earth was a hollow cell, and humanity lived on its inner surface.

Teed was nothing if not committed. He adopted the name “Koresh” (the Hebrew form of “Cyrus”), declared himself a prophet, and developed an elaborate cosmological system he called “Cellular Cosmogony.” In Teed’s model, the Earth was a sphere approximately 25,000 miles in circumference — the same as the conventional figure — but everything was inverted. The continents and oceans lined the inside of the shell. At the center of the sphere sat the Sun, which Teed described as an electromagnetic battery. The Moon and planets were reflections or refractions within the interior atmosphere. The stars were points of light produced by the interaction of electromagnetic forces at the center.

The outermost shell of the sphere was composed of layers of material — mineral, metallic, and geologic — beyond which was… nothing. Or perhaps something unknowable. The point was that the universe ended at the shell. There was no outer space, no distant galaxies, no cosmic infinity. Everything that exists was contained within the Earth-cell.

The Koreshan Unity

Teed attracted followers. In 1894, he established the Koreshan Unity community in Estero, Florida, south of Fort Myers. At its peak, the commune housed approximately 250 members who practiced communal living, celibacy (for the “inner order”), and devotion to Teed’s cosmological and spiritual teachings. The community published a newspaper, The Flaming Sword, which served as the primary vehicle for Cellular Cosmogony’s dissemination.

Teed’s followers were not uneducated eccentrics. The community included teachers, musicians, bakers, and skilled tradespeople. They built an impressively functional settlement with a bakery, a printing press, a machine shop, and extensive gardens. The Koreshan Unity was, by the standards of American utopian communities, reasonably well-organized and long-lived.

The Rectilineator Experiment

In 1897, Teed attempted what may be the most ambitious experiment in the history of fringe science: empirical proof of the concave Earth.

Working with Ulysses Grant Morrow, a geodesist and supporter, Teed constructed a device called the “rectilineator” — a series of large wooden T-shaped frames, each precisely leveled and aligned, extending along the Florida Gulf Coast near Naples. The idea was to project a perfectly straight line across the surface of the water. On a convex Earth, the line would gradually diverge upward from the water’s surface; on a concave Earth, it would converge toward the water.

Teed and Morrow reported that their measurements confirmed concavity: the straight line, they claimed, gradually approached and eventually intersected the water’s surface, proving that the Earth curved upward. They published the results with great fanfare.

The experiment had serious problems. Independent analyses have pointed out that the wooden frames would have sagged under their own weight, that thermal expansion over the course of the day would have altered their alignment, and that the measurements were conducted by believers with an obvious interest in the outcome. The experiment was never successfully replicated by independent parties.

The German Connection

Concave Earth theory developed an independent life in Germany. Karl Neupert, a pilot and engineer, proposed his own version in the early 20th century, and Fritz Braun published Ist die Erde eine Hohlkugel? (Is the Earth a Hollow Sphere?) in 1920, arguing for the concave model on geometric and optical grounds.

The most provocative claim about German Concave Earth theory involves the Nazis. A persistent story holds that in April 1942, a team of scientists sent by the German Navy to the island of Rugen conducted an experiment using infrared equipment pointed upward at a 45-degree angle — hoping to detect British naval vessels by looking across the interior of the concave Earth rather than outward. The story appears in several secondary sources, including Gerald Suster’s Hitler and the Age of Horus and Willy Ley’s writings, but primary documentation is thin. Whether the experiment actually occurred, and whether it reflected genuine belief in Concave Earth or merely a desperate wartime attempt at any available surveillance technique, remains unclear.

Key Claims

• We live on the inside of a sphere. The Earth is hollow, and human civilization exists on the concave inner surface. The sky is the interior of the sphere, and the ground curves upward in all directions.
• The Sun is at the center. A small luminous body, often described as an electromagnetic mechanism, sits at the center of the Earth-sphere and provides light and heat to the inner surface.
• Light curves. Because straight-line optics would immediately reveal the concavity, proponents argue that light travels in curved paths, bending with the electromagnetic properties of the interior atmosphere. This curved light creates the illusion of a conventional flat or convex horizon.
• There is no outer space. The universe — Sun, Moon, planets, stars — is contained within the Earth. What astronomers interpret as billions of light-years of cosmos is actually an optical illusion produced by the curvature of light within the sphere.
• Conventional physics is inverted. Gravity as understood by mainstream physics does not exist; instead, centrifugal force from the Earth’s rotation presses inhabitants against the inner surface.

Evidence

What Proponents Cite

The Rectilineator experiment. Teed’s 1897 Florida experiment remains the primary empirical claim for concavity. While the methodology has been extensively criticized, proponents maintain that the experiment demonstrated what it set out to prove.

Mathematical inversion. Some proponents, including the German theorists, argue that the concave model is mathematically equivalent to the conventional model — that you can invert the geometry of the Earth (swapping inside for outside) and reproduce all the same observations. This is technically related to a mathematical concept called “geometric inversion” or “inversion in a sphere,” and it is true that certain transformations can map a convex geometry onto a concave one. However, mathematical equivalence in an abstract sense does not mean physical equivalence — the models make different predictions about, for example, the behavior of satellites, the parallax of stars, and the physics of orbits.

Horizon observations. Like Flat Earthers, Concave Earthers claim that objects can be seen at distances that should be hidden by the curvature of a convex Earth. They interpret this as evidence that the surface curves the opposite way.

What Disproves the Theory

Satellite imagery and spaceflight. Photographs of Earth from space — taken by thousands of satellites, space stations, and missions over more than 60 years — unambiguously show a convex sphere. Concave Earth proponents must dismiss all such imagery as fabricated, requiring a conspiracy involving every space agency on Earth (including those of rival and adversarial nations).

GPS and satellite communications. The Global Positioning System works by triangulating signals from satellites in orbit around the exterior of a convex Earth. The system functions with extraordinary precision, producing locations accurate to within centimeters. This is physically impossible under the concave model without invoking additional unfalsifiable mechanisms.

Gravity and physics. Concave Earth theory requires replacing gravity with centrifugal force, but the behavior of objects on Earth’s surface — including pendulum experiments, free-fall measurements, and the behavior of fluids — is consistent with conventional gravity and inconsistent with centrifugal-force explanations on the interior of a sphere.

Stellar parallax and astronomical observation. The concave model places stars within the Earth at relatively small distances. Yet stellar parallax — the apparent shift in star positions as Earth orbits the Sun — is measurable and consistent with stars at vast distances. The concave model cannot account for these measurements without proposing additional ad hoc mechanisms for light behavior.

The Kola Superdeep Borehole. The Soviet Union drilled 12.2 km into the Earth’s crust between 1970 and 1994, encountering progressively hotter rock consistent with the conventional model of a solid Earth with increasing temperature at depth. This is incompatible with a thin shell surrounding a hollow interior.

Cultural Impact

Concave Earth theory is a historical curiosity more than a living movement, but it has had a surprisingly persistent influence on fringe cosmology. The Koreshan Unity community survived Teed’s death in 1908 (his followers expected his body to resurrect; it did not, and eventually had to be removed by county health officials) and persisted in diminished form until the last member deeded the property to the state of Florida in 1961. The Koreshan State Historic Site in Estero is now a state park, preserving the settlement’s buildings as a monument to American utopian experimentalism.

The theory has experienced a minor revival in the internet age, with small communities on YouTube, Reddit, and dedicated websites. These modern proponents typically emphasize the mathematical inversion argument and the curvature-of-light hypothesis, presenting the theory as a legitimate alternative cosmology rather than a religious revelation (as Teed framed it).

Concave Earth also serves as a useful philosophical case study in the limits of observation and the relationship between mathematical models and physical reality. The mathematical inversion argument — that you can, in a formal sense, map the conventional cosmos onto the interior of a sphere — raises genuine questions about what it means for a model to be “true” versus merely “consistent with observations.” Philosophers of science have occasionally discussed the concave Earth hypothesis for exactly this reason, not because they take it seriously as physics but because it illuminates the distinction between empirical adequacy and truth.

In Popular Culture

• Dr. Strangelove (1964) — While primarily a Flat Earth/fluoride satire, the film captures the same Cold War paranoia that surrounded all fringe cosmologies.
• Koreshan State Historic Site — The former Koreshan Unity community in Estero, Florida, is now a state park, preserving the buildings and history of America’s most organized concave Earth community.
• Edgar Rice Burroughs, Pellucidar series (1914-1963) — While Burroughs depicted a world on the inside of the Earth, his fictional Pellucidar was inspired by hollow Earth rather than concave Earth theory. Still, the imagery resonates.
• Internet communities — Small but active YouTube channels and websites maintain the concave Earth tradition, producing videos that analyze photographs and propose mathematical arguments for the inverted model.

Key Figures

Timeline

Sources & Further Reading

• Teed, Cyrus, and Ulysses Grant Morrow. The Cellular Cosmogony: The Earth a Concave Sphere. Guiding Star Publishing House, 1898.
• Mackle, Elliott J. “The Koreshan Unity in Florida, 1894-1910.” Tequesta 34 (1974): 51-70.
• Ley, Willy. “Pseudoscience in Naziland.” Astounding Science Fiction, May 1947.
• Gardner, Martin. Fads and Fallacies in the Name of Science. Dover Publications, 1957.
• Abdelkader, Mostafa. “A Geocosmos: Mapping Outer Space into a Hollow Earth.” Speculations in Science and Technology 6, no. 1 (1983): 81-89.
• Koreshan State Historic Site. Florida Department of Environmental Protection historical resources.

Related Theories

• Flat Earth — The more popular alternative cosmology claiming the Earth is a flat disc
• Hollow Earth — The theory that the Earth’s interior contains habitable spaces, civilizations, or openings at the poles