In October 2014, a research vessel from Oregon State University traveled to the Mariana Trench in the western Pacific Ocean to study the deepest part of the seafloor. The team deployed an underwater drone equipped with sensitive listening devices, which descended nearly seven miles into darkness. At that depth, water pressure reaches roughly eight tons per square inch, and sunlight has not penetrated for millions of years. A useful comparison is that Mount Everest, if placed in the trench, would not reach the bottom.
During the mission, the drone’s sensors recorded an unusual sound. It was a mix of a low moan and a sharp metallic creak, as if two different instruments had merged into a single call. The research team later named it the “bio-duck” sound. When the recording was posted online that week, no one outside the team could explain what had produced it.
The discovery came as something of a surprise because the deep ocean was long assumed to be nearly devoid of life. Scientists once described it as an underwater desert, too hostile and dark for much to survive. The footage retrieved from the trench suggested otherwise. The surveys revealed creatures not found anywhere else on Earth, including crustaceans more than a foot long and a pale, nearly transparent fish nicknamed the “ghost fish,” which appeared adapted to endure pressures about a thousand times greater than those at the surface.
The same listening equipment that captured the mysterious signal also picked up other sounds over the following months, including the low hum of ships passing above, whale calls, and the muffled roar of a storm churning on the surface miles overhead. But none of the small creatures documented in the trench could plausibly have produced a sound loud enough to be heard clearly by the distant recorder. That left investigators facing a question that neither biology nor geology could immediately answer.
For a decade, the case remained open. Researchers from several fields attempted explanations. Geologists considered the trench’s location within the Pacific Ring of Fire, which contains about 75 percent of the planet’s active volcanoes and is responsible for roughly 90 percent of recorded earthquakes. They compared the signal to seismic data from the exact time of the recording, but found no match. Earthquakes produce low-frequency sounds that stay steady in tone, while the trench recording moved across a wider and more complex range of frequencies.
Military analysts then considered whether the sound could come from underwater drones. The Mariana Trench lies relatively near Guam, which hosts one of the largest U.S. naval bases in the Pacific. Both the United States and China were known to be investing heavily in unmanned underwater vehicles, which can remain submerged for months collecting intelligence without surfacing. But when analysts examined the pattern of the signal, they found it inconsistent and irregular, repeating differently each time. Military sonar signals are designed to be regular and rhythmic, so that theory was eventually set aside.
That left a biological source as the most likely explanation, though no documented species seemed to fit. A paleontologist named Darren Nash matched part of the recording against a large library of known marine animal sounds. Certain whale species produce complex calls that can travel remarkable distances underwater, where sound moves about four times faster than through air. One such call, sometimes referred to as the “Star Wars” sound, had been attributed to dwarf minke whales in other parts of the ocean. An oceanographer named John Hildebrand pointed instead to a rarely seen animal called Bryde’s whale, noting that an existing recording taken north of the trench matched several characteristics of the mysterious signal, particularly its low-frequency opening.
But the rest of the sound, the higher and more metallic portions, matched nothing documented at the time. Bryde’s whales, which can reach more than 55 feet in length, remained one of the least studied whale species on the planet. Some researchers quietly wondered whether an undocumented species might live somewhere inside the trench, shaped by pressures and conditions no other creature had to endure.
The gap in understanding took years of patient fieldwork to close. During a month-long survey through the Mariana Islands, a team from the National Oceanic and Atmospheric Administration encountered ten groups of Bryde’s whales in the same waters where the signal had originally been recorded. Nine of those ten groups were observed making the exact call researchers had been tracking. A single sighting might have been coincidence, but nine left little room for doubt.
To confirm the finding, the team worked with Google engineers, who built an artificial intelligence system to search more than 200,000 hours of archived ocean recordings from the Mariana archipelago. The pattern matched every time. The call was consistently associated with a specific season and always traced back to one population of Bryde’s whales found only in the western Pacific. Genetic tests later confirmed that this population was biologically distinct from Bryde’s whales elsewhere.
Researchers now believe the call likely functions as a communication signal, one whale calling through open water in the hope of another responding. The exact purpose has not been confirmed with certainty, which researchers say reflects how much remains unknown about modern marine science. A sound that was once considered a possible weapon, machine, or evidence of an unknown creature turned out to be a whale quietly calling in the dark.
