1977, The unexplained extraterrestrial "Wow!" signal is detected by an Ohio State University radio telescope
In August 1977 an Ohio State University radio telescope detected an unusual pulse of radiation from somewhere near the constellation Sagittarius. The 37-second-long signal was so startling that an astronomer monitoring the data scrawled "Wow!" on the telescope's printout.
The signal was within the band of radio frequencies where transmissions are internationally banned on Earth. Furthermore, natural sources of radiation from space usually cover a wider range of frequencies.
As the nearest star in that direction is 220 million light years away, either a massive astronomical event - or intelligent aliens with a very powerful transmitter would have had to have created it. The signal remains unexplained.