On August 15, 1977, the "Big Ear" radio telescope at The* Ohio State University received a 72-second-long signal at 1420 megahertz.
Its existence was interesting enough to professor Dr. Jerry R. Ehman that he wrote "Wow!" upon the paper printout, which Big Ear had recorded several days earlier.
While unmodulated — lacking explicit information — the Wow Signal was conspicuous because it was narrow-band (and thus not "natural,") and because it was powerful.
Transmission at 1420 mhz by terrestrials is illegal by international agreement. This is because the frequency is excellent for observation of the cosmos: Earth's atmosphere and the cosmos at large is relatively transparent at its 21-centimeter wavelength, the "hydrogen line." It is for this reason, also, that 1420 would be a natural candidate for interstellar "hailing-frequency" purposes.
Calculations showed that the Wow Signal originated from beyond the Moon. It is possible that its origin was a device made by Earthlings — nations do break treaties, for example. But it did not legally come from a human-built apparatus, and it did not come from Earth unless it was deflected by an object in space.