The Wow! Signal

On August 15, 1977 astronomers at the Big Ear radio telescope in Delaware, Ohio heard a strong narrowband radio signal for 72 seconds in the constellation Sagittarius. To this day that radio pulse, known as the Wow! signal after researcher Jerry Ehman's excited scrawl on the computer printout recording it, remains the strongest evidence yet found for the existence of extraterrestrial civilizations. You can read more about it here:
The Wow! signal

Ehman's discovery is unconvincing, mainly because no trace of similar radio signals has ever been found again in the same part of the sky, or anywhere else in deep space, in the 36 years since. The signal generated excitement at the time since it matched the expectations of those searching for extraterrestrial intelligence so closely. For 72 seconds it was clear and bright, two constant tones 30 times louder than the interstellar noise in the 1.42 gigahertz range, straddling the frequency of  microwave radiation emitted by excited neutral hydrogen atoms. This is exactly what pioneering SETI researchers like Frank Drake and Carl Sagan expected a message from aliens to look like when they began the search in the early 1960s. A real civilization would presumably broadcast for longer than the length an early morning infomercial, but like Herman Cain I don't have facts to back that up.

For months after the August 15 signal the Big Ear continued to search for a civilization in Sagittarius, but heard nothing but the ordinary noise of space. Other telescopes were put to work over the years, searching the same part of the sky, but even the exquisitely sensitive Very Large Array (of Contact and 2010 fame) pondered only silence. The true nature of the signal remains poorly understood, and there are significant problems with every proposed explanation, from interstellar scintillation, to a reflection off a piece of space debris, to an alien rotating beacon. The SETI researchers and ufologists are right about at least one thing: The truth is out there. It's just really hard to discern sometimes.