Starting in the 1960s and continuing until the 1980s, sailors in Russian submarines patrolling the North Atlantic and Arctic Ocean would occasionally hear strange sounds. These underwater noises reminded the submarine crews of frog croaks, so they called the sounds “quackers” (from the Russian word for frog sounds). The sources of the sound seemed to be moving with great speed and agility; however, the submarines’ sonar (a method of detecting objects underwater) was unable to detect any solid moving objects in the area. There are several theories about what might have caused the odd sounds.

The first theory suggests that the strange noises were actually the calls of male and female orca whales during a courtship ritual. Orca whales are known to inhabit the areas where the submarines were picking up the bizarre noises. Orcas have been studied extensively, and the sounds they make when trying to attract a mate are similar to those that the submarines were detecting.

A second idea is that the sounds were caused by giant squid. Giant squid are giant marine invertebrates that live deep in the ocean and prey on large fish. They are difficult to detect by sonar because they have soft bodies with no skeleton. Not much is known about giant squid behavior, but their complex brains suggest they are intelligent animals. It is possible they have the ability to emit sound, and perhaps they approached the submarines out of curiosity.

A third theory suggests the Russian submarines were picking up stray sounds from some military technology, like another country’s submarines that were secretly patrolling the area. Perhaps the foreign submarines did not register on the sonar because they were using a kind of technology specifically designed to make them undetectable by sonar. The strange froglike sounds may have been emitted by the foreign submarines unintentionally.