The Bloop is the name given to an ultra-low frequency and extremely powerful underwater sound detected by the U.S. National Oceanic and Atmospheric Administration (NOAA) several times during the summer of 1997. The source of the sound remains unknown.

According to thw NOAA description

it rises rapidly in frequency over about one minute and was of sufficient amplitude to be heard on multiple sensors, at a range of over 5,000 km.

5000 km is a grrreeeat distance for a sound, event in the water that conduct the sounds better than air. While the audio profile of the bloop does resemble that of a living creature, the system identified it as unknown because it was far too loud for that to have been the case: it was several times louder than the loudest known biological sound.

By 2012, earlier speculation that the sound originated from a marine animal was replaced by NOAA’s description of the sound as being consistent with noises generated via non-tectonic cryoseisms originating from glacial movements such as ice calving, or through seabed gouging by ice.

The bloop sound it’s too low to be perceived by a human ear. If transposed up by a factor of 8 (3 octaves), it sounds like this.