In 1818, an American Army Officer named John Cleves Symmes, jr. spoke up a bit louder on the subject, only he proposed that the outer shell was 800 miles thick, and that there were 4 inner shells. Symmes was forceful in his conviction, and said that it was vital that an expedition be launched proposing that one be sent to the north pole to seek out an entrance he felt sure was there. The president at the time, John Quincy Adams, agreed. The trip never got off the ground, Adams leaving office before it could happen. The next president, Andrew Jackson, stopped any and all preparations for the trip. It is said that he didn't support it because he was a 'Flat Earther', but that has been proven to be false. I suspect he just saw it as being a bit 'out there'.
There were other prominent supporters of the theory, but no trips to either pole to check for any actual entrances into the Earth's interior were ever made. I guess I would say that probably the most well known hollow Earth account is that of American Navy Admiral Richard E. Byrd. His diaries supposedly tell of him flying into the Earth's interior while flying over the south Pole. There were others with him. He mentions seeing animals that looked like prehistoric mammoths. He wrote of arriving at a city known as Agartha, where he met an alien race of people who were technologically advanced. Byrd said that they were concerned about us surface dwellers because of our bombs and the way we were treating the planet.
On this subject of entrances at the poles (there are said to actually be entrances all over the planet, if they are looked for)... I found the two photos below. These were posted on the Internet by someone who says they found these 'doorways' into Agartha via Google Earth. They said that the one on the right clearly shows some sort of man-made structure. They figure it to either be some sort of manufactured retaining wall, or a "door" of sorts. Hmm...interesting idea. My thought is, if you can find a doorway into the Hollow Earth on Google Earth, then why the hell hasn't anyone already been down it? There are all of these reports/claims of seeing or knowing the location of a doorway into the Earth...if that's true, I would think someone would have checked it out by now. Don't you? One would think we would have heard more about something that huge if there were definitely entrances into a hollow Earth.
On top of the theory of our planet being Hollow, are the theories of who is living down at the end of those tunnels. Some say they are the Atlanteans. When Atlantus was crumbling around them, the Atlanteans fled into the Earth for refuge. It is believed that the land of Agartha is actually a UFO homeland where flying saucers are made and flown by Atlantean pilots. Those aren't ships from other planets with little green or grey men aboard...they are actually peaceful ships from the inside of our home.
Another 'out there' claim is about the Nazi era Thule Society and Hitler. There isn't any proof that's been found to say that Hitler was in fact a member of the group, but he did approve of it. The Thule Society (a.k.a. Study Group for Germanic Antiquity) was a German occultist group. They are most known for sponsoring the Deutsche Arbeiterpartei which became the Nazi Party. The Society were big studiers of Tibetan mythos and in turn the myths surrounding an opening into the Earth. Even as outlandish as the other claims above are, there is the belief by some that Hitler is still alive and living inside the Earth. The story is that Hitler and his chief advisers escaped the last days of the Third Reich through an opening at the South Pole (Antarctica), and are said to still be there with around 2,000 German and Italian scientists and almost a million people. Amazing what people will believe.
Jules Verne's "Journey to the Center of the Earth" might seem to illustrate that Verne believed in a hollow earth. Some do think him a prophet, interestingly enough. I don't know. The mind is a powerful thing. I suppose, one man's fictional product of imagination is another man's belief. Nothing I found confirmed that Mr. Verne was in fact a Hollow Earther.
I will admit that the idea of a Hollow Earth is intriguing. Do I believe it? Not so much. A couple of the sites I found also talked about our solar system being in a hollow planet, as well. They called it a "concave" Hollow-Earth hypothesis. Where do the originators of these ideas come up with this stuff? As I said, the mind is a powerful thing.