In 1944, American pilots flying over Germany reported seeing fast-moving Christmas-tree like objects. The objects often followed their planes, sometimes toyed with them, but never displayed hostile behavior. Still pilots shot at them, without result. Sometimes the objects vanished.
The phenomenon was so widespread that the lights earned a name, "foo-fighters," a reference to a then-popular comic strip. The military suspected that the foo fighters were a secret German weapon, until intel determined that German and Japanese pilots were seeing the same things, suspecting them of being new American aircraft.
After the war, the Robertson Panel's investigation yielded theoretical explanations including an electrostatic phenomenon like St. Elmo's Fire, reflections of light from ice crystals, or that it was in the pilots' heads. Not a man who'd seen the foo fighters was satisfied, nor was the military. The foo fighters remain a mystery to this day.
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