Signals and objects the sky has handed us that nobody has fully accounted for — the Wow! Signal, Tabby's Star, Oumuamua, BLC1, the unexplained dimming events SETI keeps logging.
On August 15, 1977, the Big Ear radio telescope picked up a 72-second narrowband burst at 1420 MHz — the hydrogen line, SETI's canonical frequency for a deliberate broadcast. The signal was 30 times background. It came from the direction of Sagittarius. It has never been heard again. Astronomer Jerry Ehman wrote 'Wow!' in the margin. Forty-eight years later, the printout is still the strongest unexplained candidate technosignature on record.