The Blue Whale is the world's largest animal.
While the pygmy sub-species is smaller than other members of the Blue Whale genus, they can still measure up to 24 metres in length.
But what makes the Chagos Pygmy Blue Whale different from other Pygmy Blue Whales is the nature of its song.
Researchers first discovered the whale when its song was picked up by underwater bomb detectors deep in the Indian Ocean.
Known as hydrophones, the instruments have been set up all over the world under the United Nation's Comprehensive Nuclear Test Ban Treaty, to monitor any illegal nuclear testing.
But they are also being used by scientists to collect data on things like seismic activity, weather monitoring and animal populations.
Scientists still don't know how many of the Chagos Pygmy Blue Whale there are, and no-one has ever actually seen one.
It's believed there are between 10,000 and 25,000 Blue Whales globally and, while they are endangered, their populations appear to be stable.
Blue Whales are notoriously difficult to study because they are shy, live far offshore, and don't often breach, like their Humpback cousins.
So it's common to use acoustic recordings to find out more about them.
