A nearly two-decade study of whale songs recorded in the Southern Ocean suggests that blue whales, the largest creatures ever to have roamed the Earth, may be recovering in Antarctica after being hunted to the edge of extinction.

Antarctic blue whales, the largest creatures on the planet, were hunted to the brink of extinction in the 20th century, researchers said. With populations believed to number in the low thousands, the animals are now considered critically endangered, according to the World Wildlife Fund.

Audio collected with underwater microphones suggests numbers at least stable after centuries of industrial whaling left only a few hundred alive

Blue whales are the largest animals on Earth, measuring up to 30 meters long and weighing up to 200 tons—as much as a Boeing 787. Yet it's the sound they make, not their size, which gives their location away.