The Earth is hollow

Don't give up, readers. We're halfway through this list. We can make it to the end. Dig deep. Well, not too deep. You see, the Earth is hollow and accessible via portals at the north and south poles. Luckily though, it's quite habitable down there, providing excellent living quarters for the lost Viking colonies of Greenland and the Nazis, while "aliens" are in fact just visitors from the subterranean yonder.