Perhaps some of the most bizarre scientific theories ever considered were those concerning the possibility that the Earth was hollow. One of the earliest of these was proposed in 1692 by Edmund Halley.

Edmund Halley was a brilliant English astronomer whose mathematical calculations pinpointed the return of the comet that bears his name. Halley was fascinated by the earth's magnetic field. He noticed the direction of the field varied slightly over time and the only way he could account for this was there existed not one, but several, magnetic fields. Halley came to believe that the Earth was hollow and within it was a second sphere with another field. In fact, to account for all the variations in the field, Halley finally proposed that the Earth was composed of some four spheres, each nestled inside another.

Halley also suggested that the interior of the Earth was populated with life and lit by a luminous atmosphere. He thought the aurora borealis, or northern lights, was caused by the escape of this gas through a thin crust at the poles.

Others picked up Halley's hollow-earth theory often adding their own twists. In the eighteen century Leonhard Euler, a Swiss mathematician, replaced the multiple spheres theory with a single hollow sphere which contained a sun 600 miles wide that provided heat and light for an advanced civilization that lived there.

Later Scottish mathematician Sir John Leslie proposed there were two inside suns (which he named Pluto and Proserpine).

One of the most ardent supporters of hollow-earth was the American John Symmes. Symmes was an ex-army officer and a business man. Symmes believed that the Earth was hollow and at the north and south poles there were entrances, 4,000 and 6,000 miles wide, respectively, that led to the interior. Symmes dedicated much of his life to advancing his theory and raising money to support an expedition to the North Pole for the purpose of exploring the inner earth. He was never successful, but after his death one of his followers, a newspaper editor named Jeremiah Reynolds, helped influence the U.S. government to send an expedition to Antarctica in 1838. While the explorers found no hole there, they did bring back convincing evidence that Antarctica was not just a polar ice cap, but the Earth's seventh continent.

In 1846 the discovery of an extinct woolly mammoth frozen in ice in Siberia was used by Marshall Gardner as evidence of a hollow earth. Gardner subscribed to the single-sun-inside-the-earth theory and suggested that the mammoth was so well-preserved because it had died recently. Gardner thought that mammoths and other extinct creatures wandered freely in the interior of the earth. This one had wandered outside by using the hole at the North Pole, then was frozen and carried to Siberia on an ice flow.

That same decade a new theory about the hollow-earth appeared. It was the brainchild of Cyrus Read Teed. Teed proposed that the Earth was a hollow sphere and that people lived on the inside of it. In the center of the sphere was the sun, which was half dark and half light. As the sun turned it gave the appearance of a sunset and sunrise. The dense atmosphere in the center of the sphere prevented observers from looking up into the sky and seeing the other side of the world. Interestingly enough, Teed's theory was hard for 19th century mathematicians to disprove based on geometry alone, since the exterior of a sphere can be mapped onto the interior with little trouble.

Teed changed his name to Koresh and founded what might today be called a cult. After buying a 300 acre tract in Florida, Koresh declared himself the messiah of a new religion. He died in 1908 without proving his ideas.

Even after his death, though, some continued to subscribe to his theory. A story is told that during World War II Hitler sent an expedition to the Baltic Island of Rugen. There Dr. Heinz Fischer pointed a telescopic camera into the sky in an attempt to photograph the British fleet across the hollow interior of a concave earth. He was apparently unsuccessful and the British fleet remained safe.

After World War II there seems to be a continuing connection between hollow-earth stories and Nazi Germany. One author, Ernst Zundel, wrote a book entitled UFOs - Nazi Secret Weapons? claiming that Hitler and his last battalion had boarded submarines at the end of the war, escaped to Argentina, and then established a base for flying saucers in the hole leading to the inside of the Earth at the South Pole. Zundel also suggested that the Nazis had originated as a separate race that had come from the inner-earth.

As time has gone on the idea of a hollow-earth has become less a theory of fringe science and more a subject of science fiction and fantasy. Perhaps this has happened because new discoveries continue to show there is no validity to most of the hollow-earth ideas. United States Navy Admiral Richard Byrd flew across the North Pole in 1926 and the South Pole in 1929 without seeing any holes leading to inner-earth. Photographs taken by astronauts in space show no entrances either. Modern geology indicates the Earth is mostly a solid mass.

One believer did seize on a NASA photograph showing a black hole at the North Pole and called it proof of an entrance to a hollow-earth.

As it turned out the photo was actually a composite of several pictures taken over 24 hours so that all sections were seen in daylight and the black hole at the top was the portion of the arctic circle never illuminated during the day over winter months.

Perhaps one of the most well-known books about hollow-earth is Jules Verne's Journey to the Center of the Earth. The book illustrates a third theory of hollow-earth which is more plausible than the other two.

This is that passages from the surface lead to caverns underground in which life thrives.

In the book three scientists climb down an inactive Iceland volcano in an attempt to find a path to the center of the Earth. They don't make it, but they do find an underground sea populated with prehistoric creatures including plesiosaurs.

Verne may have been closer to that mark than most expected. For years scientists scoffed at the idea of life thriving underground without light to provide energy. Now explorations have found rock-eating bacteria living as far as a mile below the ground. In Romania a whole ecosystem, including spiders, scorpions, leeches and millipedes has been found in a cave cut off from the surface 5.5 million years ago.

In addition to this kind of a hollow-earth there may be a "hollow Mars." A mars rock discovered in the Antarctic suggests that bacteria may have, and might continue to, exist underground on the red planet.