Beneath Antarctica's Ice

Germany sent a naval expedition in 1938 to New Swabia in Antarctica (German: Neuschwabenland) and discovered vast ice-free areas with warm fresh-water lakes, massive subterranean cavern systems, hot springs, and signs of vegetation. They began building a covert deep underground military base below the ice called and conducted research into advanced propulsion technology and saucer shaped aircraft based on alternative free energy.
The Antarctic Germans were able to develop their spacecraft far in advance of anything possessed by the U.S., and defeated a massive post WW2 military attack by Allied forces in 1946, led by Admiral Richard Byrd who retreated after suffering substantial damage and casualties in Operation Highjump, which is still classified as Top Secret.