The "Wow!" Signal.
Astronomers have speculated for years that, if intelligent alien life were real, they would have similar or better understandings of physical sciences as we do. One thing that is universal in chemistry and physical sciences is frequencies emitted by certain atomic elements. Therefore, intelligent alien life would most likely have an understanding of these frequencies as we do. So, if intelligent life wanted to communicate in a way which was universal between planets (i.e. not a special language or numeric system), then emitting a signal at a certain requency would be an excellent way of doing that. As hydrogen is the building block of all other elements -- due to its simplicity and integral nature -- astronomers speculate that, if intelligent life wanted to communicate through frequencies, they would do so in the hydrogen frequency.
In the 70's, radio telescopes in the US were pointed up towards the sky collecting any radio data that came in. Most of the data collected are seemingly nothing, just random radio noise. However, in the early morning of August 15, 1977, one of these radio telescope centers began to get a TON of data. And, it was all at or near the frequency of hydrogen. And, the signal was strong and fairly consistent, meaning that it would be incredibly unlikely it was random radio noise or passing radiation.
Technicians monitoring the data were dumbfounded by the signal, and had no explanation for why it was coming or how it was so consistent. So, they just wrote "Wow!" on the data sheet.
We still don't know how such a powerful signal could have even penetrated our atmosphere and remain consistently transmitted for as long as it did. Some people believe it could have been military testing nearby, but, again, it would be incredibly unlikely such a consistent message could be transmitted through simple interference (plus, no military testing was reported in the surrounding area).
The same or similar data has not been reported since, leading many to believe that the single could have been an "S.O.S." signal from a dying planet, or just a passing coincidence that we may never see again.