In August 1977, a sky survey conducted with Ohio State University's "Big Ear" radio telescope found what has become known as the 'Wow' signal. Registering an enormous signal strength (60 Janskys in a 10 KHz channel, which is more than 50 thousand times more incoming energy than the minimum signal that would register as a hit for today's Project Phoenix.). The shape of the signal had the characteristic rise and fall expected for its short 72 second lifetime. But a hitch remains: the signal has not been retrieved from other sky surveys, making it more anomaly than confirmable cosmic source. This odd, one-beam behavior could be caused by an alien transmission that simply went off the air during the 3 minutes between beams.

If the putative aliens permanently shut down their transmitter, then there's no chance of ever hearing the Wow signal again. Like a single sighting of the Loch Ness monster, we would never be able to prove what it was. But if the signal is periodic - if, for example, the aliens are using a rotating radio beacon that sweeps the star-studded strata of the Milky Way once every five minutes or every five hours - then we could hope to find it by just looking again.

For the moment, despite the use of the Harvard META SETI system, the Very Large Array (VLA) and the new 26-meter radio telescope in Hobart, Tasmania, Wow has not been detected again.