What do you know about the hollow Earth and the hollow Moon?
A hollow Earth is a recurrent concept through history by many cultures, communities, and thinkers, each one giving its own color, texture, and folklore. For many people this idea may be a crazy concept that deserves no attention at all. However, even great scientists have embraced this concept, or some similar, or even a craziest one.
Halley and the earth's four poles
Near the decade of 1690, Edmund Halley1 (1656-1742) set forth the idea of a hollow earth. For Halley, the earth's interior was more than a simple big empty sphere. The earth's interior was a series of concentric spheres diminishing in size but keeping a proportion to our nearest planet orbits.
A comparison between Kepler's harmony among planet orbits from the Sun outwards, and Halley's concentric spheres toward the center of the earth.
He though that the outer sphere --the one we live in-- was about 500 miles deep. Below that there was another spherical crust in proportion to the orbit of Mars, then further below was another sphere, and further below was the "sphere of Mercury", maybe enclosing the deepest hollow earth's core.
The use of the concept of concentric spheres to explain the physical surrounding universe was not new at that epoch. Near a century before Halley's system, Kepler had devised a marvelous planetary harmony for the planets of the solar system. Halley's intrepid idea came when he was trying to devise an explanation for the behavior the magnetic polar lines. He went so far as to postulate that there are four magnetic poles.
So ancient is the idea that the earth must be hollow at its center that it is attributed to thinkers like Plato, Aristotle, Lucretius, Seneca, and others. The basis for that reasoning was the presence of springs, volcanoes, deep caves and caverns. In addition to that, the presence of frequent fossils suggested the idea that the earth's interior must also be populated. Keeping in mind the limited knowledge that they had at that time, their reasoning was no so bad.
People from the earth's interior
After Halley there was no mayor interest to the idea of a hollow earth. Next to him, the big jump came from John Cleves Symmes, but in the interim appeared a novel from the Norwegian writer Ludvig Holberg (1684-1754).
Niels Klim's Underground Travels2, is the title of a satirical novel he published where the main character falls down to the earth's center where he finds another sun. Down there he finds that the people are talking tree-like creatures with many arms and faces below the branches.
Cleves Symmes wanted to go to the earth's center
Halley believed that the earth was hollow, but made no effort to go and see it with his own eyes.
On the contrary, 
John Cleves Symmes, Jr. (1779-1829), stated the following,
I declare the earth is hollow, habitable within; containing a number of solid concentric spheres; one within the other, and that it is open at the pole twelve or sixteen degrees. I pledge my life in support of this truth, and am ready to explore the hollow if the world will support and aid me in the undertaking.
Symmes was clearly defined about his ideas and was determined to explore them. He wrote1:
I ask one hundred brave companions, well equipped, to start from Siberia, in the fall season, with Reindeer and slays, on the ice of the frozen sea: I engage we find a warm and rich land, stocked with thrifty vegetables and animals if not men, on reaching one degree north-ward of latitude 62; we will return in the succeeding spring.
Symmes was very serious about his ideas. He traveled a lot spreading them and looking for support for his expedition.
Symmes was an eager reader and collector of everything related to geography, explorations and expeditions. His proposition of a hollow earth gained him a thousand jokes, but at the same time he gained the support of some well-know gentlemen of relevant reputation.
On the other side of the world, "Count Romanoff, Chancellor of Russia under Czar Alexander, was planning a polar expedition, had heard of Symmes's theory, and had requested the captain's services". Symmes turned this request to his favor and boosted the attendance to his lectures.
Symmes did a great job pushing and influencing science fiction writers like Edgar Alan Poe The Narrative of Arthur Gordon Pym of Nantucket (1838), Jules Verne - A Journey to the Centre of the Earth (1864), Edgar Rice Burroughs - At the Earth's Core (1914), among others.
Symmes's theory also influenced many other schools of thought like the New Age mysticism, and UFO research.
We live inside the Earth!
The mother of all out-out-this-world ideas related with the hollow Earth came from Cyrus Reed Teed (1839-1908) when he proclaimed that he had the revelation that the universe is inside the Earth, so the Earth's crust. According to Teed's theory, we live on the inside of the planet, not the outside.
In Cyrus' theory, gravity is non-existent. We are held in place not by gravity, but by centrifugal forces.
Gravity thus does not exist, and humans are held in place due to centrifugal force. The sun is a giant battery-operated contraption, and the stars mere refractions of its light.
The universe of Cyrus Teed is a solid rock hollow cell 8000 miles in diameter. We live and walk inside the spherical inner surface of this cell. Therefore, our heads point toward its center. The entire universe that we is within this cell, cradled "in the hands of God."
Nothing can be more anti-Copernican than this model. He recreated the physics in such a way that light rays travel in curves, instead of straight lines.
William Reed: a beautiful world inside our hollow Earth
In 1906, William Reed published his book The Phantom of the Poles where he questioned the existence of the Earth poles. His book was written to address the following questions that he posed in the Introduction to his book:
The problems to be solved are as follows:
1. Why is the earth flattened at the poles?
2. Why have the poles never been reached?
3. Why is the sun invisible so long in winter near the farthest points north or south?
4. What is the Aurora Borealis?
5. Where are icebergs formed, and how?
6. What produces a tidal wave?
7. Why do meteors fall more frequently near the poles, and whence do they come?
S. What causes the great ice-pressure in the Arctic Ocean during still tide and calm weather?
9. Why is there colored snow in the Arctic region?
10. Why is it warmer near the poles than six hundred to one thousand miles away from them?
11. Why is ice in the Arctic Ocean frequently filled with rock, gravel, sand, etc.?
In the Conclusion of his book he makes the final arguments in favor of the Earth being hollow:
The earth is either hollow or it is not. What proof have we concerning the latter? Not one iota, positive or circumstantial. On the contrary, everything points to its being hollow.
At his times maybe there were no irrefutable proofs of the Earth not being hollow, but we cannot take this as a proof that it was hollow:
If it be so, and there are burning volcanoes in the interior, would you not see great lights reflected on the icebergs and the clouds just as any other great fires reflect the light? Would not great clouds of smoke and dust be seen?---the same as from any other burning volcano? That is what all the explorers have witnessed--low, dark clouds rising out of the ocean, or at the edge of the ice. Nansen says: "Let us go home; what have we here to stay for? Nothing but dust, dust, dust." Where could such dust come from--so bad that it was one of the great annoyances in the heart of the Arctic Ocean--if it did not come from an exploding, burning volcano? If the earth be hollow, would it not be warmer in winter and cooler in summer? Arctic explorers say that a north wind in winter raises the temperature, while a south wind lowers it. As an opposite fact, in summer a south wind raises the temperature and a north wind lowers it. This is just what would occur if the winds came from the interior of the earth.
Again: if the earth be hollow, it could not he round, inasmuch as the opening would take from its roundness in proportion to the size of the opening. All now agree that the earth is flattened at the poles. Also, it is warmer the farther one goes north or south. Why is this the case?
This is the case because the Earth is not absolutely solid. If the Earth be solid, there would be any polar flattening, but having a polar flattening does not prove that it is not completely solid and that, in consequence, it should be hollow.
There is but one answer, and that is that the earth is hollow, and is warmer in the interior than on the exterior. As the wind passes out in the winter it warms the atmosphere. If the earth be solid, neither science nor reason furnishes any rational theory why it should be warmer as one passes north. Every known theory is against such a conclusion.
The behavior of the atmospheric and undersea currents is a very complex. Tales from seamen are not enough to prove or disprove any theory.
As soon as you adopt the belief that the earth is hollow, perplexing questions will be easily solved, the mind will be satisfied, and the triumph of sensible reasoning will come as a delight never to be forgotten.
Its a beautiful world the one he describes! Maybe somewhere else in the universe there are hollow planets and hollow moons. We cannot discard any possibility.
I like to make an odd comparison between Einstein, Planck, and William Reed. It's an uneven and maybe capricious comparison you may think, but my motivation is the following: Einstein loved to study the Universe in its macro version, the Universe as whole; Plank was absorbed in studying the Universe from the tiniest particle up; but Reed wanted to know what is below our feet. The first two were dry and separated from the society; Reed was warm and conveyed the society's need for keeping the fantasy alive. Einstein and Plank were interested in their peer's papers and publication, Reed was interested in the lives and travels of the adventuresome sailors that were capable of giving their lives for a cause or a fantasy. William Reed is not a scientist in the sense of Einstein or Plank; his name cannot be found in the books about the history of science. But who owns the science? Who owns the history of science? Science is not property of anybody. The strict scientific method is not the only way of building knowledge. Science is a cultural activity, a cultural achievement, so everybody that tries to understand the physical universe that surrounds us should deserve a position, or at least a mention, in the list of those who have contributions to this intellectual field.
Russian postal stamp depicting Neznaika, the anti-hero boy that went to the interior of the Moon from the didactic children's story: Neznaika on the Moon.
Notes
1. For a detailed account of Halley's ideas, and how he repeated Kepler's cosmology, see The subterranean world of Edmund Halley.
2. Almost all about Symmes and how his theory influenced the Mormon Church can be found at Oliver Cowdery' bookshelf: John C. Symmes (1780-1829) Hollow Earth Theory.
Footnotes
1. See Hollow and habitable within: Symmes's theory of earth's internal structure and polar geography.
2. The novel by Louis Holberg Journey Under The Ground can be downloaded for free from this Website. Go to the E-Books catalog for the complete list of free books available.