New research shows the largest animals on Earth blue whales. And this is such good news. They're making their home in a part of the Indian Ocean where they were wiped out decades ago. Victoria Gill has more.

The biggest animals on Earth, blue whales have returned to inhabit tropical waters where, just a few decades ago, they were wiped out. On two recent expeditions, researchers and documentary filmmakers came to the Seychelles in search.

Of the giant marine mammals.

This is really cool. I've never seen so many different species, so many of so many different species. We've got hundreds of animals here. It's pretty remarkable.

This is now an ecological paradise teeming with life. But back in the 1960s, soviet whaling vessels captured and killed hundreds of blue whales in these waters.

All right. Here you go.

Okay.

To find out if the animals had returned, scientists listened. Underwater microphones are able to pick up the very low frequency sounds that blue.

Whales use to communicate over distances of.

Hundreds of miles through the ocean. Them. But the researchers weren't actually able to hear the blue whales calling directly. It took a year of recording from the seabed and painstaking analysis of all those months of sound to identify the telltale very low frequency deep calls.

The signature of these.

Victoria Gilbert. You're watching BBC News.