From about.com, unverified:
Hitler’s Nazis were convinced that they were destined to rule the world, and they came to this warped conclusion through the acceptance of many occult beliefs and practices, including astrology, the prophecies of Nostradamus, and the hollow/inverted Earth theory… hohlweltlehre. Because they suspected that our surface is on the interior of a concave Earth, Hitler sent an expedition, including Dr. Heinz Fischer and powerful telescopic cameras, to the Baltic island of Rugen to spy on the British fleet. Fischer did so not by aiming his cameras across the waters, but by pointing them up to peer across the atmosphere to the Atlantic Ocean. The expedition was a failure, of course. Fischer’s cameras saw nothing but sky, and the British fleet remained safe.
This is probably apocryphal, but every time I run into this story I start laughing again at the earnest Nazis peering into deep space trying to see what the Allies are up to. Meanwhile, of course, we’re building nuclear weapons with all the Jews and Slavs who had to get the fuck out of town.