In 1989, a sonar surveillance system listening for Soviet submarines picked up a strange sound in the Pacific Ocean: a low, repetitive vibration with a frequency of 52 hertz. Navy officers initially attributed it to a machine, but eventually decided it must have come from a living creature. As for what kind, they didn’t know.

By that time, the Cold War was drawing to a close, and the Navy soon deemed it safe to share the data with unaffiliated scientists. Navy technician Joe George got in touch with William A. Watkins, a leading expert in marine mammal bioacoustics, hoping he might be able to solve the mystery of the unfamiliar thrum.

52 Is the Loneliest Number

From 1992 until his death in 2004, Watkins and his colleagues at the Woods Hole Oceanographic Institution tracked the calls, which showed up in the North Pacific every year between August and February. In a paper published in the December 2004 issue of Deep Sea Research Part I: Oceanographic Research Papers, the researchers concluded that it seemed like a whale was responsible. Because the calls were always isolated, and they didn’t overlap with the movement of other baleen whales in the area—specifically blue, fin, and humpback—the 52-hertz whale (or just “52 Hertz”) appeared to be something of a solitary figure.

“Obviously, he’s able to eat and live and cruise around,” study co-author Mary Ann Daher told The Washington Post. “Is he successful reproductively? I haven’t the vaguest idea. Nobody can answer those questions. Is he lonely? I hate to attach human emotions like that. Do whales get lonely? I don’t know. I don’t even want to touch that topic.”

While Daher and other researchers resisted making assumptions about the creature, the public latched onto the idea of the whale as an outcast, swimming alone and singing a tune that its fellow whales either couldn’t understand or simply wouldn’t respond to. Since 2004, the so-called “loneliest whale in the world” has become both mascot and muse for those who feel friendless or misunderstood. It’s inspired books, sculptures, tattoos, and other artistic tributes; even BTS released a song, “Whalien 52,” about it in 2015.

But for all its emotional resonance, there’s quite a lot we don’t know about the whale—like, for example, whether it’s actually a whale. Scientists generally agree that chances are good, and the leading theory is that it’s a hybrid of two whale species, perhaps blue and fin. Blue whale calls fall between 10 and 39 hertz, and fin whales usually vocalize in pulses at either 20 or 40 hertz. Blue-fin hybrids are a documented phenomenon, but their calls are not, so it’s possible that they sing at a slightly higher frequency than their parents.

To try to prove (or disprove) the theories surrounding 52 Hertz, filmmaker Joshua Zeman embarked on a journey to locate the animal itself. He chronicled his endeavors in the recent documentary The Loneliest Whale: The Search for 52, available to stream now.

Channeling Captain Ahab

Before beginning the search, Zeman first had to secure sponsors for his project—easier said than done, considering he couldn’t guarantee an outcome.

“When we pitched the story, some places were like, ‘Oh yeah, that’s great; I love the story. But can you produce the whale?’ They would finance it if we knew we were going to find the whale,” Zeman told The Washington Post. “But what happens when we find the whale? What are we going to do? Are we going to hug it? The power is in the metaphor.”

Zeman eventually reeled in a couple celebrities involved in ocean conservation efforts: Leonardo DiCaprio and Entourage’s Adrian Grenier. His next task was to enlist a team of experts who knew enough about tracking whales and analyzing their calls to give Zeman a fighting chance of finding a never-before-seen one. John Hildebrand, a professor of oceanography at the University of California San Diego’s Scripps Institution of Oceanography, not only brought his expertise in whale song to the mission, but also an invaluable lead. One of his interns had detected 52 Hertz’s call in California’s Santa Barbara Channel, suggesting that the whale might be still alive and relatively close by.

John Calambokidis, a senior research biologist and co-founder of the Cascadia Research Collective, came on board, too, as did Ana Širović, an associate professor of marine biology at Texas A&M University at Galveston. In October 2015, Zeman, the scientists, and other crew members set off on a week-long voyage around the channel to hunt for 52 Hertz. Using audio data from 1000-pound “sonobuoys” dropped in the water and drone footage, they hoped to locate whales in the area. Then, Calambokidis and his cohorts would head out in a smaller boat and tag the whales with suction-cup devices that captured both audio and video footage. If they happened to tag 52 Hertz, they’d likely be able to identify it.

An Ocean of Mystery

(From left to right) Joshua Zeman, John Calambokidis, John Hildebrand, and Ana Širović.Bleecker Street

Since finding 52 Hertz was the primary goal of the entire venture, we’ll leave it up to the documentary to reveal whether it was achieved. But the film isn’t just about that single investigation. It explains how the discovery of whale song helped people view them as intelligent beings, fueling the 1960s movement to end commercial whaling. A current threat to marine ecosystems also gets some screen time: noise pollution from shipping, oil extraction, and other human activities.

The participating scientists had other aims for the mission, too. “I knew I could get some useful data and information, whether we find the particular whale we were searching for or not,” Širović said in a press release. “We were able to record some cool footage of a blue whale singing underwater, which was the first such recording [obtained from tag data] which corroborated our previous understanding of how they call.”

Hildebrand and Calambokidis had already been researching blue whale song together, so this particular trip was a continuation of that work. “We incrementally collected more data on blue whales in the Santa Barbara Channel,” Hildebrand tells Mental Floss.

When it comes to whale communication, however, humans are still mostly in the dark. “The more we learn about whale song, the more idiosyncratic it appears,” Hildebrand says. “We don't know much about how whales respond to the calls of other whales. One theory is that females use the calls of males to select them for breeding, but that is just a theory. No one has seen a female react to the call of a male.”

It’s also common for blue and fin whales to call when no other whales are around, Hildebrand explains. In short, 52 Hertz’s frequency is indeed higher than the usual frequency of blue and fin whale song; but it may seem so abnormal mainly because our understanding of what’s normal is based on limited data. And there is a lot more to we still need to understand. “Incredibly,” Hildebrand says, “new species of baleen whale are still being discovered.”