Once upon a time, there was a sound. The sound was picked up by the United States National Oceanic and Atmospheric Administration, or NOAA, sometime in 1997. The NOAA was tracking oceanic sounds and picked up on this one specific sound. You can hear it above.
What's strange about this sound, however, is that scientists feel confident that it's from an animal. The problem is that the sound came from roughly 5000 kilometers away, meaning that the animal in question, theoretically, would have had to have been larger than a blue whale.
What the bloop is, we don't know. I'm betting on Cthuhlu.