There are several hollow Earth theories. The most prevalent one holds that there are great but hidden openings at both the North and South poles, and that it is possible to enter those holes. Some - including the respected Admiral Byrd - claimed to have entered those holes. This hohlweltlehre theory was a failure: because they suspected that our surface is on the interior of a concave Earth, Hitler sent an expedition to the Baltic island of Rugen to spy on the British fleetand it failed.

Via paranormal:

Then there's the legend... that Hitler and many of his Nazi minions escaped Germany in the closing days of World War II and fled to Antarctica where at the South Pole 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. This story gets more complicated with Nazi-designed UFOs, Nazi collaboration with the people who live in the center of the Earth, and the explanation for "Aryan-looking" UFO pilots.

...Totally crazy...