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When Andréa Thiebault, a researcher at Nelson Mandela University in South Africa, spent a month in 2019 watching videos from penguin-borne cameras, she was trying to better understand how the seabirds hunt and catch their prey. But she also ran into a surprise: the first recordings of seabirds making sounds underwater. “I couldn’t believe it,” Thiebault recalls. “I had to replay it many times.”

On land, penguins are noisy. They make and listen for calls to find their mates or chicks in crowded colonies. Some also communicate from the sea surface, probably to help them form groups during foraging trips. Given their vocal skills and the amount of time they spend diving, researchers had long suspected that penguins could call underwater. But until recently nobody had reported hearing them.

Penguins are small birds that dive deep, so Thiebault’s research team needed a lightweight camera that could withstand high pressure to spy on them. Her colleague David Green attached small action cameras to the backs of eight king, three macaroni, and 14 gentoo penguins, and sent about 10 hours of footage to Thiebault. When she finished analyzing the videos, Thiebault had counted 203 underwater vocalizations coming from all three species, though not from every penguin they spied on. Many of the calls sounded like short whoops.

Exactly why the penguins make these sounds remains a mystery. In the video, the calls are extremely short—about 0.06 seconds on average—and the penguins tended to be alone when making them, so Thiebault says it is unlikely they were trying to communicate with each other. She notes that these penguins only called while hunting, sometimes within seconds of speeding up in the water or just before catching fish. That might help explain why underwater vocalizations haven’t been heard from penguins in captivity, which are typically fed dead prey. The team wonders if penguins in the wild might be using the sounds to stun their food.

Hannah Kriesell, a biologist from the Norwegian University of Science and Technology who studies the acoustic communication of marine animals, says it will be interesting to follow up on the purpose of these calls. It might be possible, for example, to play such sounds underwater in a controlled setting and see how prey responds.

Knowing that penguins produce sound underwater “opens the door for a lot more research,” says Kriesell, who was not involved in the study.

For now, says Thiebault, “just the fact that we discovered this behavior is intriguing.”