During the cold war, the American and Soviet navies put huge efforts into hiding their submarines, and into detecting the other side’s submarines, and as a result, there were huge advances in the technology required for listening for any signals from the ocean depths. And the two navies heard some unusual things.

From the mid-1970s onwards, Soviet submarine crews started to report strange sounds in the waters of the Arctic and North Atlantic oceans. These high-pitched, frog-like noises were a mystery to the crews, who started to refer to them as Quackers (Квакеры). It was assumed by many in the Soviet navy that these noises were being made by a secret NATO submarine-detection technology, but at the same time, this explanation didn’t make any obvious military sense (what’s the point of a detection technology that also makes noise?) and didn’t explain why the noises themselves seemed to be similar to more familiar animal-like sounds. The submarine’s crews also inferred (through Doppler shift measurement) that some of these noises were being produced by objects travelling at over 200 kilometers (125 miles) an hour. To this day, the sources of the sounds remain unknown.

On the other side of the world, the United States Navy Equatorial Pacific Ocean autonomous hydrophone array had been listening out for Soviet  (and later Russian) submarines since the mid 1960s, and in 1997 they recorded a powerful ultra-low frequency underwater sound with no obvious source. The sound, which they called the Bloop, was heard at a range of over 5,000 kilometers, and is several times louder than the blue whale, the loudest known underwater animal sound. Though there are various theories about what might have caused the Bloop (including some kind of giant squid — an explanation also occasionally offered about Quackers), there is no comprehensive explanation, and the source  of the sound, like the Quackers, remains unknown.

Listen to the Bloop (sped up sixteen times):

Or take a listen to the real-time recording here.

Though there are no easily-available recordings of Quackers, they have been extensively reported on within Russia, including this (slightly kitschy) Russian-language TV documentary, Quackers, The Military Evidence:

There were many other strange sounds first heard by human ears as a result of this frenetic submarine-chasing cold war activity, but the Quackers and the Bloop remain, for now, mysterious.