The Wow! signal
Scientists, by nature, are usually even tempered and slow to be impressed. That's what happens when you devote your life to thorough, careful examination and weighing of real-world observations. So, in 1977, when a researcher at Ohio State caught a signal that made him write down the word "WOW!" the scientific community took notice.
For over 50 years, scientists have speculated that, should aliens want to communicate with us, they might choose to use radio signals. Radio waves are cheap to produce and travel great distances across space. Astronomers also predict that aliens would probably use a radio wavelength of 1420 MHz, because that is what happens when you zap a hydrogen atom (and hydrogen is the most abundant element in the universe).
Knowing that, imagine the excitement when an observer got a strong, 2.5 minute radio signal that just so happened to be 1,420 MHz. Scientists anxiously awaited a follow up broadcast, perhaps some sort of transmission or message, but aliens have left them waiting by the phone longer than a stood-up date on prom night. Mysteriously, the signal was never repeated, leaving scientists waiting for the past 37 years.