Here's another one then, and this one has crackpot written all over it.
The Earth is hollow - For centuries, people believed that the earth is hollow in the center, that it is not a superheated mantle spinning around a solid iron core, but is in fact a hollow sphere of crust, like a gumball. Edmond Halley theorized it in 1692, that the Earth is a shell, 500 miles thick, with concentric rings inside. Inside there would be a solid core, and between the rings there would be various atmospheres of fluorescent gases. When these gases escape at the poles, they create the aurora borealis and australis.
By the 20th Century, it had reached the mainstream, been argued against by scientists and caught the attention of Adolf Hitler, who sent a mission to the South Pole to find the entrance he was sure was there. It would lead to the underground, where Hitler believed a subterranean race of 12-foot-tall, super intelligent humanoids lived. He intended to convince them to come up to the surface and fight for Nazi Germany, or at least give the Nazis some futuristic firepower. Whether Hitler sent this mission or not is part of the theory, but if he did, it never came back. This was due to either having been killed by the subterranean race, or Antarctica.