Continuing the alien theme started by Curzon:
Close to midnight on August 5, 1952, four American air traffic controllers walking across the tarmac at Haneda Airport (then a US military base) spotted a round, bright object in the sky over Tokyo Bay. They went up to the tower and took a look through their binoculars, and noticed a larger dark ellipse surrounding the light.
Over the next few minutes, the controllers tried to get visual confirmation from an airborne observer plane, which couldn’t see anything. They were able to get a radar fix on the UFO, though, and so they had a scrambled fighter jet intercept it. The pilots didn’t spot the UFO, though, and shortly after the radar intercept the UFO disappeared.
The original US Air Force report is available in scanned format here. Nobody was ever able to explain what happened; my personal theory is that the aliens were coming for Kenzo Tange so they would have someone to do their design bidding on Earth.