In 2007, a man planned to go on a voyage to the North Pole to find the "gateway" to the center of the Earth, spurred on by his understanding of the "Hollow Earth" theory which had been disproved in 1774.
Back before we really knew what was going on beneath our feet, there were theories. In 1692, astronomer Edmond Halley speculated in a paper that the Earth was hollow. Looking at the Earth's magnetic field, Halley noticed that it was shifting and variable. He believed that this was because the Earth was hollow, and the magnetic fields were being thrown off by three inner "shells" within the Earth, each with its own magnetic poles.
Halley imagined the objections of critics of the theory, including that the outer shells could crumble and descend with gravity onto the inner circle, revealing the surface to the mole people below, essentially like this:
Halley believed that the shells were “lined throughout with a Magnetical Matter, or rather to be one great Concave Magnet” and so the shell is kept up, given that magnetism is a stronger force than gravity.
Really running wild with the theory, Halley proposed that there may be life down there, arguing that otherwise, it would be pointless.
"Since it is now take for granted that the Earth is one of the Planets, and they all are with reason supposed Habitable…Why then should we think it strange that [this] prodigious Mass of Matter…should…serve [only] to support its Surface? Why may not we rather suppose that [it]…is so disposed by the Almighty Wisdom as to yield as great a Surface for the use of living Creatures as can consist with the conveniencey and security of the whole," he wrote, adding "we ourselves, in Cities where we are pressed for room, commonly build many Stories, one over the other, and thereby accommodate a much greater multitude of Inhabitants”.
Halley, and other later proponents of the Hollow Earth theory, were wrong. Halley at least was working with incomplete information – with better data on the Earth's magnetic field, he might not have felt the need to explain it by having the Earth become an onion.
There are many variations of Hollow Earth theory, some of which propose – of course, without evidence – everything from there being an underground Kingdom called Agartha, or that there are Nazis lurking inside the hollow space and biding their time. It's not known how either group would get things like "vitamin D" or "food" in this scenario. Some versions say that we are living on the inside, and the stars we see are on the other side of the shell
They have of course all been disproved. We have a pretty good idea of what is going on inside our planet, bar for a few mysterious and ancient structures lurking within. The Earth is not hollow, but made up of the crust, mantle, and an outer and inner core.
The first experiment to prove the Earth was not hollow took place in 1774, when scientists used a mountain in Scotland to calculate the density of the Earth. The team showed that the sheer mass of mount Schiehallion attracted pendulums towards it. Using this and surveying the mountain, they were able to calculate a rough density of the Earth.
We now know that the Earth has a density of about 5.5 grams per cubic centimeter (3.2 ounces per cubic inch), Andrew Campbell, a professor of geophysics at the University of Chicago explained to Politifact, while the average rocks we've found at the crust weigh 2.7 grams per cubic centimeter (1.6 ounces per cubic inch) on average. Given this discrepancy, we can assume there is matter below us, and it's a lot denser than the surface.
Since the mountain experiment, we have come up with much more sophisticated ways of looking inside our planet, utilizing earthquakes in a technique known as seismic tomography. When earthquakes occur, waves of energy are sent out in all directions. By measuring the tremors from several locations at the surface, scientists can create a map of the Earth's interior. Since rocks and liquids within the Earth are of different densities, the waves move through them at different speeds, allowing geologists to figure out what type of material the waves are going through. We have found some cool and unusual features using this method, but no hollow spots or mysterious underground kingdoms.
Nevertheless, in 2007, one author of a Hollow Earth book planned to journey to the North Pole to find the secret entrance using an ice-breaking ship, but the project was canceled, much to the dismay of the ice moles, living on the shell below.