Though the Hollow Earth has been a popular idea for fiction for decades, I was intrigued to find that there were many who held it to be based on fact. There are a variety of religious groups of the internet who quote extensive biblical and other references which they feel point to a Hollow Earth. There was even an exploration planned for this year 26th June-9th July, organised by a man called Steve Currey. The plan was to travel to 84.4 N Latitude, 141 E Longitude and they had charted a Russian icebreaker 'Yamal' to take them there along with up to 100 people who applied at http://www.voyagehollowearth.com/ Unfortunately Currey died before the mission began and it was cancelled.
Throughout the 19th and 20th centuries there have been reports of people travelling in the Polar regions discovering warm winds and other indications that there is a more temperate location up (or down) there. There is disagreement about what the worlds inside our Earth are like. A key difficulty for people is how it would be lit. One could envisage there may be deep underground caverns, possibly even with entrances at the North and South Poles as some believe (plus other entrances such as Mato Grosso in Brazil and Mount Epomeo in Italy even under the Egyptian pyramids) but they would be rather dark and bleak places, whereas most who believe in the Hollow Earth think there are rich, advanced civilizations down there. The Nazis believed in the theory of the inverted or hollow Earth, termed 'Hohlweltlehre'. The mission to Ruegen Island in April 1942 by Dr. Heinz Fischer was focused on the belief that we were actually on the inside of a concave world. Ironically Nazi Germany's developments in rockets would lead to disproof of that. There are myths of Nazi leaders (presumably led by Martin Borman who disappeared) escaping with 2000 scientists from Germany and Italy who were apparently missing at the end of the Second World War and up to 1 million other people into the interior of the world. This line has been followed in the story, 'Indiana Jones and the Hollow Earth' by Max McCoy (2003) which features Indiana and the Nazis racing to find a hidden civilization within the Earth.
There is a group called the International Society for the Complete Earth (ISCE) (http://www.hollow-earth.org/welcome.html) which was set up in 1977 by a former German Second World War submarine commander who, amongst other things, apparently, helped recover the Holy Lance in 1969 and assisted the Hartmann Expedition to Antarctica in 1979 to recover a treasure of Hitler's. The trouble with things on the internet is that this could be a real group (it claims 1000 members across 9 countries) or it could be entirely a spoof for a book or a movie or something, it is so difficult to tell. If it is not a spoof then it would be a great organisation to feature in some modern technology version of 'The Da Vinci Code'.
As I discussed with the inland sea of Australia, in an age when we know all about the World people remain keen to find out that there is more, something that has been overlooked, and certainly in the expectation of advanced civilizations, societies or models of society which are fairer or more decent than our own (or far less decent if they are made up of old Nazis) as an element of hope in our violent selfish world.
A website called Xenophillia has 5 pages of different evidence which proves that a Hollow Earth is impossible: http://www.xenophilia.com/zb0008.htm To me, putting aside issues of the density of the Earth and its structure, there seem basic logistical problems of civilizations inside Earth which appear to be as follows. Without sunlight there cannot be plants and so no food supply for animals or humans to live. I know molluscs can live at volcanic vents deep in the sea and there are blind cave fish but you would need a lot to sustain more than a handful of people for a long time. Authors have suggested that the magma inside the Earth would provide light like the sun, but it is not known to do so in any deep caves any humans have visited. Without plants there would be a lack of oxygen and any creatures would soon die from the carbon dioxide they would generate. I know there are supposed to be large vents but basically any settlement would have to be ground around these and then why not live on the surface where all of this stuff is so much easier? We are always discovering new species, they thought the coelacanth was extinct but then they caught one in 1938 again in 1952 off South Africa and in 1998 off Sumatra. Then in 2005 remains of the so-called 'hobbit' race alive in Indonesia up to 18,000 years ago, about 12,000 years after the Neanderthals and Homo Erectus had gone. So, I am not saying there are not mysteries to be discovered, but that the logistics of a hollow Earth just do not add up. The Earth is probably about 80% the size it was once it first gained a crust due to internal cooling. There is also evidence to suggest it has expanded and contracted throughout its history suggesting it is a ball of molten rock at least gradually cooling. Below are illustrations of different views of how people think a hollow Earth would work:
This model below comes from John C. Symmes (1780-1829) who, like Edmund Halley (1656-1742 - discoverer of Halley's Comet) envisaged a series of concentric spheres inside the Earth:
Photos from Apollo 8 and Apollo 16 suggest to some people that there is a big chip in the North Pole which opens into the interior. This certainly makes Earth look like a soft-boiled egg. There is also a map of where people think the entrance will be: