A powerful alien probe that came to Earth in the 23rd century to contact humpback whales, threatening the planet when no whales answered.
The probe was a huge cylindrical object of unknown origin. As it neared Earth, its signal drained power from starships and Starfleet facilities and began vaporizing the oceans, causing great storms. Spock determined the signal was humpback whale song, aimed at a species that had since gone extinct.
In Star Trek IV: The Voyage Home, Kirk and his crew traveled back to the late 20th century and returned with two humpback whales, George and Gracie. The whales answered the probe's call, and it restored power and departed.