Antarctica

This mini-documentary from Strange But True Stories gives an account of the anomalous details of the US Navy's large-scale 1947 Operation Highjump led by Rear Admiral Byrd (Ret), that conducted scientific experiments and aerial reconnaissance of Antarctica. The end goal was to establish US sovereignty over a sizable chunk of the continent but the planned 6-month mission was cut short after just few weeks. Antarctica has been integral to the body of paranormal folklore spawned by the National Security Act ever since.
There have been persistent legends of an underground Nazi base in the Antarctic region of Neuschwabenland, of extraterrestrial bases; of these two groups working together and of their violent encounters with the US Navy, some with their roots in the strange reports on Byrd's mission published in Chilean newspapers during that time.
This video cites a "secret diary" alleged to be Byrd's that contains an account which purports to explain his 3-hour absence during a sortie, an event that actually did precipitate the early cancellation of the Operation. An edition of the diary can be bought today, with added comments by Timothy Beckley, William Reed, Tim Swartz – and the famous "Commander X". This diary has apparently been circulating since the 1970s and functions as a kind of Bible for believers in the Hollow Earth.
In this "diary", Byrd describes flying over a green pastureland surrounded by mountains and an outside temperature of 75ºF (24ºC), when his plane's controls stopped working and its flight became controlled by an external force. He looked out and saw that he was being escorted by flying discs emblazoned with swastikas. His plane was safely landed and he had an encounter a Nordic "master", who told him that with the discovery of nuclear energy, mankind's demise was inevitable but that his people would still be here to seed the next civilization, which would hopefully understand the futility of war.
The 1947 National Security Act created fertile ground for urban legends like the one in this "secret diary" to take root but it would be wrong to say that it is the driver of persistent rumors about Antarctica, which have been ramping up over the past few years.