Between 1956 and 1961, The Ohio State University built a large radio observatory at nearby Delaware OH. Known as Big Ear, it had a reflecting surface roughly the size of a football field, with antennae at the end zones. Big Ear was used in the Search for Extraterrestrial Intelligence (SETI) from 1973-1995, longer than any other facility in the world. It was “tuned” to 1420 megahertz, because hydrogen (the most abundant element in the universe) emits that frequency; thus an advanced civilization would likely be aware of it. Signals at 1420 MHz would presumably have an extraterrestrial origin, since that frequency was reserved for SETI, and earth-based facilities were prohibited from transmitting it. On this date in 1977, Big Ear detected a strong signal that appeared to come from the direction of Sagittarius. Astronomer Jerry Ehman circled the data on his computer printout and wrote Wow! in red ink. This observation has been known as the Wow! signal ever since. A plot of signal intensity (averaged over ten-second intervals) vs. time shows it rising and falling smoothly over a 72-second window. The alphanumeric sequence 6EQUJ5 is simply its signal-to-noise ratio with E=15; Q=26; etc. Ehman never observed a comparable signal again, despite more than fifty attempts. Nothing remotely like the Wow! signal was detected during two decades of searching at several facilities, including the Very Large Array in New Mexico, which is much more sensitive than Big Ear. 6EQUJ5 remains, to this day, our strongest physical evidence for a possible extraterrestrial civilization.