August 15, 1977. The Big Ear radio telescope at Ohio State University was scanning the sky for signs of extraterrestrial intelligence. At 11:16 PM, it captured a signal so intense it spiked the recording equipment for exactly 72 seconds. The signal, originating from the constellation Sagittarius, was a narrowband transmission on the 1420 MHz frequency. This specific frequency, known as the "Hydrogen Line," is a protected band where artificial signals would be most logically transmitted by an advanced civilization. Jerry Ehman, the astronomer processing the data days later, noted that the signal showed the classic signs of an artificial origin rather than a natural source like a pulsar or a quasar. It didn't fluctuate in the way natural background noise does. Ehman famously grabbed a red pen and circled the alphanumeric sequence "6EQUJ5" on the printout, scrawling "Wow!" next to it. That brief, handwritten exclamation became the name of the most enduring mystery in SETI history. In the years following, researchers have pointed the world's most powerful telescopes at the same coordinates repeatedly, but the signal has never returned. While some skeptical researchers later proposed a comet-based origin, the intensity and structure of the transmission still defy conventional scientific explanation, leaving the door ajar for the possibility that we heard someone—or something—calling out across the cosmic void.