There has been much speculation about the real state of the earth. The Hollow Earth theory has been around a long time, many people have tried to prove it, quite a few films made about this topic.
Here is one theory by Raymond Bernard, who wrote a book called "The Hollow Earth".
In it he states:

Our world is hollow, with the crust of the earth being 800 miles thick. There exists two openings at the North and South Pole, each hole having a circumference of 1400 miles wide. At the centre of the earth is not a molten core but an inner sun which is six hundred miles wide and is 2900 miles from the Inner Surfaces. The diameter of the lip at the opening at the poles is 1200 miles long, thus a person can not see the other side of the opening.
There exists three worlds on our planet, the outer surface, where we live, the middle earth which purportedly is lined with many caverns, tunnels (made by someone), Inner Cities and people who live there and lastly the Inner Surface.
How Gravity works then is the following. For the people who live on the outer surface, Gravity holds them down. For the people that live in the middle earth, the closer they get to the centre of the crust (i.e. - 400 miles down), the less effect gravity will have upon them.
Furthermore Bernard states that the Aurora Borealis that is only visible up north, is the reflection of the inner sun off clouds passing over the opening of the pole. Also that some explorers of the North Pole have discovered the closer they get to the Pole, the warmer it gets instead of the colder.

An explorer called Admiral Byrd in 1947, went to explore the North Pole. He states that he actually entered into the Inner Earth at the opening at this pole. He also states, he made contact with an advanced people in this area who showed him their world and asked for him to take a message back to the surface dwellers and government of their concern about a future vision they had of our destroying our world and since this is their home as well, they of course wanted to prevent this. If such a people do exist, it is theorized that possibly some of the UFOs we see are not extraterrestrial but intraterrestials, as our elder brothers keep an eye on us.
If you would like to read extracts of his diary I include the link:
http://www.v-j-enterprises.com/byrdiar.html

More will be posted on this topic. :tup:

Another great topic, Chris! :tsup:

A little bit more information here to add to this topic.

Adolph Hitler even gets in on this topic. Some of Hitler's top advisors - perhaps even Hitler, believed that the Earth was hollow; and there was at least one expedition by the Nazi military to exploit that belief for strategic advantage during the war.
There's a legend, that Hitler and many of his Nazi staff, escaped Germany in the closing days of World War II and fled to the South Pole, where they had discovered an entrance to the Earth's interior. According to the Hollow Earth Research Society in Ontario, Canada, they are still there. After the war, the organization claims, the Allies discovered that more than 2,000 scientists from Germany and Italy had vanished, along with almost a million people, to the land beyond the South Pole.

As with all such stories, it's often difficult to sort out facts, exaggerations, and outright fabrications. But it is a story that is worth investigating further.

There is another theory named the "Inverted earth" theory. This one claims we are actually living on the inside of the earth, we are held not by gravity, but by centrifugal force, as the earth rotates. The celestial bodies are twinkling lumps of ice and the illusion of day and night is caused by a rotating central sun that is half brilliant, half dark!

More to ponder! :Ohmy:

Interesting additions! Thanks for the effort. :tup:

I strive to be openminded as much as possible, but in this instance, I have to say that I have read a bit about the inverted earth theory and even I can't give it much probability at all.

Ditto on the part of the theory about the Nazis escaping to the land beyond the South Pole.

Mind you I am enjoying hearing all theories though!

There are actually 4 main hollow earth theories:

The Concentric Spheres Hollow Earth Model
Edmund Halley(English astronomer - Halley's comet) in 1692, came up with this theory while trying to understand the Earth's magnetic field. In order to explain the complex movements of the field, Halley concluded that there must be at least 4 concentric shells each with their own magnetic properties.

The Polar Holes Hollow Earth Model
This theory states that at the North and south pole, there are openings between 2000 and 4000 kilometres across that open to the interior of the planet.In this theory the centre of the planet harbours a central sun that provides light and heat to the world within.

The Inverted Earth Hollow Earth Model
Cyrus Reed Teed, an herbalist and self-proclaimed alchemist, had a vision of a woman in 1869, who told him that we are living on the inside of the hollow Earth. For nearly forty years, Teed promoted his idea in pamphlets and speeches. He even founded a cult called the Koreshans (Koresh is the Hebrew equivalent of Cyrus).
It works on the principle that we are actually living inside a hollow planet right now and the centre of the planet is a point infinitely far away. All the other planets, the moon, satellites and the sun revolve around this central point.

The Complete Shell Hollow Earth Model
Based on a combination of the hollow Earth and expanding Earth theories, it provides an alternate explanation for the drifting continents phenomenon making the Plate tectonics theory obsolete.
This theory shows how the accumulation of matter during the planet forming process naturally produces a planet structured differently to what is currently theorised. It is also shown how a planet hollows out and expands under its own gravitational power.

I also note that an expedition from the US is due to search for a way in to the inner earth next year.