On the evening of August 15th, 1977, the now-legendary 'Wow!' signal was picked up by a radio telescope at Ohio State University and has baffled scientists ever since.
Jerry Ehman was the man behind the notorious note which called attention to something weird that he noticed in the data when he studied it a few days later and scribbled 'Wow!' next to the circled numbers on the readout.
The signal stood out as something which defied explanation and, since the goal of the project was to search for ET messages, it instantly became, almost by default at the time, the best possible clue that intelligent life may be out there trying to contact us.
However, several attempts over the years to find the signal again proved fruitless and only lead to the 'Wow!' moment becoming an even bigger enigma to astronomers.
Over the years, numerous theories have been put forward as to the source of the signal, including a controversial hypothesis advanced a few months ago that the event was caused by comets.
While seemingly rather promising, even that theory was met with disagreement from astronomers, leaving the 'Wow!' signal firmly entrenched in the realm of mystery on its 40th anniversary.
Since that its been four decades since the signal was detected, one would assume that it was some kind of one-off celestial event and not an ET beacon.
Then again, until and unless the mystery is solved once and for all, there remains a sliver of hope that just maybe the 'Wow!' signal was a long distance call from an ET race that's still waiting to hear back from us.