A faint pop punctuated the sound of crashing waves—the first hint something was amiss.
Sitting on board the R.V. Falkor in December 2014, David Barclay heard the sound through headphones plugged into an underwater microphone on the ship's hull. His mind flashed to the pair of scientific instruments sinking through the water beneath his feet, en route to an abyss in the Pacific Ocean known as Challenger Deep. The spot lies nearly seven miles below the waves—more than a mile deeper than Mount Everest is tall—making it the ocean's deepest point.
The two instruments were part of Barclay's work to create a compact and less expensive way to record the underwater soundscape, a project he began as a graduate student at Scripps Institution of Oceanography. Studying the thrum of the sea could not only help scientists understand the ocean’s structure but also aid in picking out particular melodies, whether from whales or submarines.
The instruments’ round trip to record inside Challenger Deep was expected to take about nine hours. But when the time came, only one survivor returned from the depths.
As Barclay later deduced, the pop came from the implosion of one instrument's glass housing, a 15-inch-wide sphere that encased the electronics. Though the instrument was destroyed, Barclay and his colleagues eventually found useful refrains within the cacophony of the collapse. The team used bouncing sound waves from the implosion, recorded by the surviving device, to calculate one of the most precise measurements ever taken of Challenger Deep.
Past measurements largely cluster between 10,900 and 10,950 meters, but the new estimate is among the deepest yet: a whopping 10,983 meters, or 36,033 feet.
Scientists have long known that Challenger Deep is the ocean's deepest point, but they've spent decades working to pin down precisely where and how far down its deepest spot lies.
"We like to know the extremes of the planet," says study author Scott Loranger, an acoustical oceanographer at Woods Hole Oceanographic Institution. "What's the tallest mountain? What's the driest desert? What's the spiciest pepper?" Such efforts help expand the limits of human knowledge, he adds. "At its most basic, that's what every scientist is trying to do."
Barclay, who is now an associate professor at Dalhousie University in Nova Scotia, describes himself as "professionally paranoid"—a natural disposition after a career spent tossing expensive equipment off ships. This paranoia leads him to meticulous preparation. The night before each big deployment, he writes up a list of everything that could go wrong.
"It sounds a bit sadistic," he says. "But it's a really good exercise to do."
Enumerating the possible routes to failure helps Barclay avoid human-caused catastrophes, such as forgetting to charge an instrument battery or failing to turn on a device. However, there are always things that are out of Barclay's control.
Exploring our planet's greatest depths isn't easy. Miles of water overhead create crushing pressure, which bears down in Challenger Deep at eight tons per square inch, or roughly a thousand times the pressure at the surface.
Only a few people have ever visited Challenger Deep. The first were Swiss oceanographer Jacques Piccard and United States Navy Lieutenant Don Walsh, who descended on board the bathyscaphe submersible Trieste on January 23, 1960. As the Trieste neared the seafloor, the plunging temperature fractured a plexiglass window, sending a loud crack through the cramped cabin. But the window held, and Piccard and Walsh arrived safely in Challenger Deep, lingering for 20 minutes before ascending.
The pair was programmed to descend to a certain depth and linger, recording the oceanic symphony before returning to the surface. One, known as Deep Sound Mark II, would travel to 9,000 meters down. The other, Deep Sound Mark III, was intended to reach the ocean bottom. But once they disappeared from sight, there were few ways to track their progress.
"You pull the rope and release it, and it's gone," Barclay says. "You don't see it. You don't talk to it. You don't know what it's doing for that whole time."
Ever prepared, Barclay had set up the ship's onboard underwater microphone to record at the surface, occasionally listening in for clues to what was happening below. That's when he heard the pop. That evening, still unsure of what had happened, he and the crew peered out across the ocean surface at the appointed hour to retrieve the instruments. They found only one bobbing in the waves.
The scientists heaved Deep Sound Mark II back onto the ship and listened to its recording. A rush of noise cut through the silence, a cacophony created by the implosion below of the Mark III. Barclay speculates that one of the instrument’s small ceramic floats may have failed, triggering a cascade of destruction.
As the instrument's glass collapsed under the weight of five miles of water, it released a pocket of air that oscillated under pressure before splintering into a veil of tiny bubbles. The sound from all this activity raced through the water, careening off the surface then returning back to the depths of the ocean—where Mark II was listening.
"It became very obvious right away that it was gone," Barclay says of the destroyed instrument.
The bouncing waves
Six years later, Loranger sat at his desk listening to the reverberations from the blast. With field work stymied by the pandemic, he was hoping to find something useful in the recording. After an initial chaotic slew of sound, several distinct echoes can be heard, each slightly quieter than the last until they peter out into silence.
"I forgot to hit stop, and I'm just typing away," Loranger says. That’s when he heard something strange. Some 25 seconds after the implosion, a faint "pew" cut into the recording. The echo had traveled almost 25 miles, bouncing between the surface and the ocean's deepest point multiple times. "Holy crap," he recalls thinking. "I didn't expect that at all."
Measuring sound waves is one of the most common ways to map the seafloor, just like a bat using echolocation to see in the dark. For many years, researchers detonated explosives at or near the water's surface to generate sound to bounce off the bottom. More recently, scientists have shifted to more controlled methods of creating noise, such as pressurized air, says Mark Rognstad, an expert in seafloor mapping at the Hawaiʻi Institute of Geophysics and Planetology who is not part of the study team.
The deeper the water, the more intense—and lower pitched—the sound needs to be to reach the bottom. The 2014 implosion provided just this source of intense sound. The collapse of glass housings under pressure can be quite violent, Rognstad says. He was part of an expedition funded by National Geographic to search for ships sunk during the World War II Battle of Midway when the implosion of a glass sphere within a remotely operated vehicle wreaked havoc. "It was described to me as like a stick of dynamite going off," he says.
The intensity of the Mark III implosion sent shockwaves zipping back and forth between the surface and seafloor, which was key to the team's precise measurement. Using the acoustic features of one of the echoes as a template, Loranger and his colleagues identified the arrival times for the initial loud burst and each reflection. The researchers then modeled the paths of the various sound waves, adjusting for changes in the speed of sound at different depths caused by shifts in temperature, pressure, and salinity.
This led them to their final calculation for the depth of Challenger Deep: 10,983 meters, with a margin of error of plus or minus six meters.
The magic of sound
Different methods have produced varying numbers for the depth at Challenger Deep—and as technologies advance, the efforts to find the ocean’s greatest depths will surely continue. A meticulous analysis published just last year produced a depth of 10,935 meters from acoustic and pressure measurements collected during the dives of Victor Vescovo in the submersible Limiting Factor. Some variation is to be expected between different methods, as each has their own challenges and uncertainties.
"There's no way to put a ruler and measure exactly," says Rochelle Wigley, a geologist with the University of New Hampshire who wasn't part of the study team. She points out that the difference between the two most recent values isn't that large at less than 0.5 percent.
Regardless of what the exact depth of Challenger Deep may be, part of the fascination is in the hunt—and the many unknown wonders that might be discovered along the way. The implosion itself is an example of happenstance discovery, producing data that the researchers never set out to collect.
The recording also offers everyone a way to experience a place that few will ever have the opportunity to visit, Barclay says. Like shouting into the Grand Canyon to listen to its echoes, the recording is "a way of really being in the trench," he says. "And that, I think, is really magical."
As for Barclay's plan to listen to the soundscapes within Challenger Deep, he and his colleagues finally accomplished that goal. In 2021 they landed a device at our planet's deepest point, recording the tranquil rhythms of the ocean for four hours.