Quacker

Quackers (Rus. "квакер" -- an word based on a Russian rendition of frog sounds) are mysterious sounds, similar to a frog noise, widely reported by the crews of  submarines from various parts of the  and  during the peak of the Cold War, as well as their assumed sources.

Cold War
During the Cold War, when Soviet went to patrol northern seas, their crews started reporting the mysterious frog-like sounds, which soon were dubbed "quackers", from the Russian rendition of a frog noise. These sounds appeared when submarines passed certain zones in the sea and behaved as if they were emitted by some moving underwater object that failed to register on the active sonar. When the sub left their "patrol zone", the objects disappeared after emitting one final "quack".

These objects exhibited behavior not unlike some marine animal or manned vessel, showing obvious interest in the passing submarine, circling around it, trying to actively avoid sonar pulses, and so on. The speed of some of these objects (estimated from Doppler shift of their sound frequency) was in the range of 200 km/h, much higher than any then-known man-made vessel. was attempted on several occasions, but, apart from some obvious reactions to these attempts (such as changing the pitch of the sounds or movement of the apparent sound source), nothing came of it.

The peak of quacker observations occurred at the end of the 1970s, when the areas where the sounds appeared started to multiply and spread over from the Barents Sea to other areas including the North Sea and the North Atlantic in general. The Soviet Academy of Sciences was invited to create a joint commission with the representatives of the Navy, as this phenomenon was identified as a potential national security risk. This commission worked for about a decade, but despite extensive investigations results remained inconclusive, and it was eventually disbanded. In the 1980s the phenomenon slowly faded, and now quackers may have disappeared completely.

Proposed explanations
There never was much consensus about the nature and origin of these sounds, and the only hard fact about them is that they existed. Official reports of the commission remains classified to the present day, even if it was known that it never reached a conclusion. Several hypotheses were proposed, but none reached full acceptance, as they all failed to account for at least some of the phenomenon's properties. The three main ideas about the origin of this phenomenon propose some secret new technology developed by the US or NATO, an unknown marine animal or extraterrestrial activity.

Secret technology
A secret technology was the most popular hypothesis explaining this phenomenon. During the height of the Cold War these mysterious happenings greatly affected the morale of submarine crews, and as such were seen as direct threats, even if they exhibited no hostile intent. It was initially believed that these sounds were sonar pulses from American fixed sonar barrages, analogous to passive SOSUS network. However, it soon became apparent that their sources were mobile and active, so they were then believed to be small tracking subs, deployed to keep an eye over Soviet boats' movements.

Some experts are said to hold this opinion to this day, although apparent speed, mobility, and noise level of these "subs" (that are moving absolutely silently and could be only identified by their "quacks") are still unmatched for any known man-made vessel. They also failed to be observed by active sonars, despite numerous efforts, also casting doubts on their human origin. Also, while such technology appears at least hypothetically possible for current engineering and manufacturing techniques, the cost of it seems to be prohibitively high, even for the most affluent nations like the U.S.

Unidentified marine animal
Marine animals are another proposed source of these sounds and now are viewed as the most probable hypothesis. At first, it was ascribed to the orca's mating calls, which sound rather similar, but orcas usually mate on the surface or close to it, while quackers were always observed at depths of no less than 200 m. Other cetaceans were also proposed as a candidate, both living and extinct, such as the ancient somewhat snake-like whale Basilosaurus. Basilosaurs are of special interest to cryptozoologists, who propose that they are still alive, and sea serpents of marine lore are true sightings of these animals. Their purported habitat also roughly matched areas of quackers observations.

Another animal that could be responsible for it would be some type of giant squid of the Architeuthidae family. That would explain the quackers' active behaviour, as squids are known to possess a high level of intelligence and could possibly mistake subs for their eternal rivals, sperm whales. They are also deep-sea creatures, and are reported to have mobility matching some of the quackers. Being cephalopods, they also lack rigid internal skeleton, and that might contribute to their invisibility to sonars. However, squids do possess an internal shell remnant known as a gladius or pen. Squids also do not have lungs or vocal folds, and so might be unable to emit sound underwater, as a whale or dolphin (both mammalian) could.

These hypotheses, however, fail to account for the dynamic of the phenomenon, which slowly started in the mid-60s, grew in frequency and area through the 70s, when quackers' "patrol zones" began to appear as far as around Greenland. While they were originally observed only in the Barents Sea, they slowly faded out in the 80s disappeared by the end of century. Matching this in living animals requires enormous behavioral changes to happen species-wide and almost immediately, no less than two times, which is unheard of in modern biology.

Extraterrestrials
Extraterrestrial activities were also proposed as an explanation, but this claim was already quite dubious in the 1960s, so it received the least attention of all three major ideas. No evidence were collected towards this end. The said conjectures are patrol-like pattern of quacker behavior, as if they tried to protect their underwater base, their active interest in the submarines, and mobility unmatched by human vessels. Dynamics of their observation would also match the life-cycle of a large Cold War monitoring project that was eventually closed as tensions dropped and nuclear exchange became less and less probable.