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: