Blue whales may be the biggest animals in the world, but they are also some of the hardest to find. Not only are they rare they are also reclusive by nature and can cover vast areas of ocean.

However, a team of scientists led by the University of New South Wales (UNSW) Sydney, Australia, are confident they’ve discovered a new population of pygmy blue whales, the smallest subspecies of blue whales, in the Indian Ocean.

And it was the whales’ powerful singing – recorded by underwater bomb detectors – that gave them away.

“We’ve found a whole new group of pygmy blue whales right in the middle of the Indian Ocean,” says UNSW professor Tracey Rogers, marine ecologist and senior author of the study.

“We don’t know how many whales are in this group, but we suspect it’s a lot by the enormous number of calls we hear.”

The discovery was made possible using data from the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), an organisation that monitors international nuclear bomb testing.

Since 2002, the CTBTO have been using advanced underwater microphones called hydrophones to detect soundwaves from potential nuclear bomb tests. The recordings – which pick up many other detailed ocean sounds – are available to scientists to use for marine science research.

The UNSW-led team were studying the data when they found an unusually strong signal: a whale song that had previously been identified in the recordings, but that scientists still knew little about. After closely studying its composition - details like the song’s structure, frequency and tempo - they realised that it belonged to a group of pygmy blue whales but not the ones previously recorded in the area.

“I think it’s pretty cool that the same system that keeps the world safe from nuclear bombs allows us to find new whale populations, which long-term can help us study the health of the marine environment,” says Professor Rogers.

Pygmy blue whales are the smallest members of the blue whale family, but that is the only small thing about them: they can reach up to 24 metres long, which is almost the length of two standard buses.

If visual sightings confirm this new population, they would become the fifth population of pygmy blue whales to be discovered in the Indian Ocean.

“Blue whales in the Southern Hemisphere are difficult to study because they live offshore and don’t jump around – they're not show-ponies like the humpback whales,” says Rogers.

“Without these audio recordings, we’d have no idea there was this huge population of blue whales out in the middle of the equatorial Indian Ocean.”

The team named the newly-found population Chagos, after the archipelago – island group - they were detected nearby. While the team are confident in their findings, it is impossible to confirm the species without a visual observation. Visual sightings for such an elusive animal can be tricky and expensive to fund, so it is unlikely this will be verified anytime soon.

The finding is big news for marine conservation, as blue whales were brought to the edge of extinction after whaling in the 20th century. And unlike many other types of whales in the Southern Hemisphere, their numbers have not sprung back.

Rogers is now leading a team using the CTBTO data to study how the Chagos population has changed over time. The findings could teach us how the whales adapted to warming ocean temperatures over the past 18 years – and how they might fare moving into the future.

“The largest animal in the world is one of the hardest ones to actually study,” says Rogers. “There are many more of these blue whales out there than we’ve realised – and we’ve only been able to find them with the help of this international infrastructure.”