Whale song is a form of communication made by the male Humpback whale of Earth. The male Humpback will sing his song anywhere from 6 to as long as 30 minutes and then start again. In the ocean, other whales will pick up his song and pass it on. The songs change every year, and until the 23rd century, served an unknown purpose. Long speculated as some sort of navigational signal or as part of a mating ritual, the truth behind the whale song was not determined until long after their extinction.
In 2286, an unknown alien probe entered the Terran solar system emitting a high frequency transmission, focused on Earth's oceans. It was later determined that the probe and its transmission were, in fact, whale song. (Star Trek IV: The Voyage Home)