A vast cylindrical alien probe of unknown origin that arrived at Earth in 2286 calling to humpback whales, and whose signal crippled starships and began boiling the oceans when no whale answered.
The probe is the threat in Star Trek IV: The Voyage Home. It crosses Federation space broadcasting a powerful signal that drains the power of everything it passes, including the starship Saratoga. On reaching Earth it aims its transmissions at the seas, ionising the atmosphere, covering the planet in cloud and starting to vaporise the oceans.
Aboard a captured Klingon Bird-of-Prey, Spock recognises the signal as the song of humpback whales, a species hunted to extinction in the 21st century. The probe's makers had evidently been in contact with whales long before and had come to find out why they fell silent.
Kirk's crew slingshot round the Sun to 1986 San Francisco and, with the help of marine biologist Gillian Taylor, bring back two humpbacks, George and Gracie. Released into San Francisco Bay, the whales answer. The probe falls silent and leaves, its builders never revealed.