In 1997, the U.S. NOAA's Equatorial Pacific Ocean autonomous hydrophone array detected a "bloop" sound. The "bloop" was consistent with the sounds of an animal, but it was far too loud. It could have been aliens. Or a sea monster. Or aliens. But it's not.
Somewhat disappointingly, NOAA is now saying that the "bloop" (which you can listen to here) is consistent with the sound of an icequake in a large iceberg, probably located off the coast of Cape Adare in Antarctica. This is in contrast to NOAA's original speculations, which focused on some sort of gigantic mythical animal. Sigh.
But fear not (or resume being afraid), because there are still several mysterious underwater sounds that NOAA can't fathom, including a mysterious whistle and a series of upsweeping sounds loud enough to be heard throughout the Pacific that have been going on since NOAA turned its hydrophones on back in 1991. You can listen to "whistle" here, and "upsweep" here.