They came from the CENTRE of the earth...

Some of Hitler's top advisors believed that the Earth was hollow; and there was at least one expedition by the Nazi military to exploit that belief for strategic advantage during the war.  Many believe that the interior of the Earth is home to strange races of technologically advanced beings. 

There are several hollow Earth theories... The most prevalent one being that there are  hidden openings at both the North and South poles, through which it is possible to enter.  Admiral Byrd, a respected man, was claimed to have entered via these holes.  Warmed and lit by an interior sun, it is believed that there are other civilizations that live within the Earth on it's inner surface. 

This idea has led writers to create various novels based on this topic; Edgar Allen Poe (MS Found in a Bottle), Edgar Rice Borroughs (At the Earth's Core), and Jules Verne (A Journey to the Center of the Earth).