During the cold war, listeners in the deepest recesses explored by manned submarines would occasionally hear mysterious chirping sounds, dubbed "quackers" while scanning for soviet sounds. The strange anomalies were sometimes attributed to enemy subs, but there are several other theories that there was something even stranger going on beneath the waves. There have been a number of theories proposing potential sources of the objects including extraterrestrials.
With extraterrestrials on the list, it's hard to imagine anything stranger being proposed and investigated as the official cause of a mysterious anomaly overheard by sonar operators. The strange hums and chatters did sound almost like some sort of alien creature chirping. At least it's easy enough to imagine such a thing while hearing the strange sounds.
The theory often proposed with quackers in conjunction with the UFO phenomenon was that these chirping sounds were the manifestation of some underwater technology being utilized. Explanations varied from the engines themselves to a form of communication to a malfunctioning device that cloaked the alien submersible ships. With reports even in the 1960's pouring in from newspapers and tabloids the phenomena no doubt raised a few eyebrows from believers, but very little evidence was ever gathered to suggest any relation to extraterrestrials.
Another theory was that the strange sounds were the manifestation of deep sea creatures that had never been documented or seen before. Stories of giant squid latching onto ships and sinking them were considered wild myths by the majority of officers aboard these submarines. Even today, though the largest squid officially documented far exceeds the largest known of 1960, there is still no known creature that creates these chatters so commonly observed in radar stations.
And another odd component of the phenomena is how suddenly it all vanished in the mid 1980's. Though there are still rumors of the sounds reaching the ears of deep sea listeners these newer cases are often highly suspect. No confirmed hearings have occurred in several years. So whatever the quacks were, they were specific to the Cold War. This has led many to speculate that the incidents may be related exclusively to a military technology that was deployed during that period but no other. While the case has never been solved, there is still no shortage of theories on what precisely these mysterious quacks could have been.
There is perhaps one final thing worth mentioning in the mysterious case of these phantom sounds. In the cold war there were mysterious radio signals known as number stations that would transmit information out in the open in ways that could not be decoded by any but those who had the keys on the other side. Is it possible this chirping could have somehow been related to the number stations by sending messages encoded in what sounded to the untrained eye like chirping sound? Such a code would have been extremely difficult to decode as not only would it require new coded language, but it would also be almost entirely unidentifiable as a language in the first place. Perhaps it sounds like a stretch, but as the quacks sound, this is one Cold War phenomenon that remains to this day entirely unexplainable.