Inside the hollow Earth are aliens, Nazis, and the Lost Tribes of Israel
The shell of the Earth is about 800 miles thick before it opens up to a massive hollow containing its own sun. Entry points are at the North and South Poles. The residents include the Lost Ten Tribes of Israel, led inside by God, where they live for hundreds of years and enjoy such advances as "flying saucer technology." This according to Rodney Cluff, author of World Top Secret: Our Earth IS Hollow!: The Scientific, Scriptural and Historical Evidence that Our Earth Is Hollow!
Over at Atlas Obscura, Eric Grundhauser interviewed Cluff for a wonderful feature about the fictional and non-fictional (?) history of the hollow Earth:
One of the most popular pieces of evidence for Hollow Earth is a supposed secret journal entry by Admiral Richard Byrd, who claimed to be the first person to fly over the North and South pole. According to believers, Byrd's secret journal from 1947 included a report of flying into one of the Symmes Holes, and making contact with the race that lives inside the Earth.
Of course this too, has supposedly been covered up.
Through the mid-2000s and into the early 2010s, Cluff was actually a part of a long-gestating expedition known most recently as the North Pole Inner Earth Expedition. Unfortunately after a number of setbacks including backers and members of the team falling victim to calamities ranging from cancer to fatal plane crashes, the expedition was put on hiatus. Had the expedition been successful, the team would have chartered one of the world's largest ice breaking ships straight to the North Pole, where they would have attempted to contact the denizens of the Hollow Earth through the hole they believed they would find. Cluff believes that the various setbacks to the project are the work of the international banking conspiracy, but is hopeful that they will someday be able to get funding, and a new expedition leader to help continue the project.
"The Hollow Earth is filled with giants, Germans, and a little sun"