In the early 19th century an American eccentric, John Cleves Symmes (1779-1829), sought funding for an expedition to enter the Earth through one of two 4,000-mile wide polar holes. Inside the Earth, he was convinced, a benevolent advanced civilization existed.
Though an object of derision to most people, some took him seriously, and the idea of a hollow Earth was championed in a number of books throughout the rest of the century and right into the next.
There is even a strong Nazi wing of the movement. According to Canadian neo-Nazi Ernst Zundel, the principal advocate of this theory, Hitler and his elite troops escaped with their saucer technology into the hole at the South Pole.