The first, and most famous, potential E.T. signal is the so-called Wow! signal. SETI researcher Dr. Jerry Ehman found the signal in 1977 with Ohio State University's Big Ear radio telescope. The signal appeared as a coded set of intensity readings, 6EQUJ5, which Ehman circled and wrote, "Wow!" next to on the printout. It was a notable signal because it was much more intense than galactic background noise, and was found very close to the frequency astronomers expect to find alien transmissions, 1420 MHz. That's the frequency hydrogen atoms resonate at, and hydrogen is the most common element in the universe.

What happened after the signal was found? Not much. Scientists tried to find the signal again to help pinpoint its origin, but no one has ever been able to track it down. It was apparently not a continuous signal, because only one of the Big Ear's two detectors caught it. Over the ensuing decades, hours and hours of telescope time have been devoted to scanning that area of the sky, and nothing like the Wow! signal has been heard there since. It's still a very mysterious thing, because most terrestrial origins have been ruled out. That leaves secret military or government technology, or a natural phenomenon in space that we haven't found yet. Or aliens.

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