Sound Speed, Ship Speed and Beam Width: What Really Determines the Depth of the Challenger Deep

The Challenger Deep, at the southern end of the Mariana Trench, is the deepest known place in the ocean, and it has been measured repeatedly since the middle of the twentieth century. Yet even in the era of satellite positioning and modern multibeam sonar, published estimates of its maximum depth still disagree with one another by several meters, and in some cases by more than ten meters. A new study led by researchers at the National Institute of Polar Research, the Graduate University for Advanced Studies (SOKENDAI), the University of Tokyo and partner Japanese institutions asks a deceptively simple question: when the seafloor lies nearly 11,000 meters below the surface, what actually controls the number a survey reports as the depth? The answer, the study shows, is not a single instrument reading but a chain of assumptions about seawater, sound, ship handling and data processing.

The team mapped the Challenger Deep using the EM124 multibeam echosounder aboard the Japanese research vessel Hakuho-maru, acquiring dense acoustic coverage of the western, central and eastern basins of the deep. Rather than announcing a new record, the researchers deliberately treated their own dataset as an experiment in measurement. They reprocessed the identical acoustic returns through five different seawater sound-speed models and compared the resulting maximum gridded depths. In the eastern basin, the deepest of the three, the estimates ranged from 10,914 meters to 10,932 meters. That 18-meter spread, produced from one set of echo data, reveals how sensitive a full-ocean-depth measurement is to the way sound is assumed to travel through the water column.

The physical principle behind acoustic bathymetry is straightforward. Depth is obtained by multiplying the speed of sound in seawater by the round-trip travel time of an acoustic pulse and dividing by two. At Challenger Deep depths, sound needs roughly fifteen seconds to travel from the ship to the seafloor and back again. The complication is that sound speed is not a constant: it varies with temperature, salinity and pressure, all of which change dramatically between the surface and a trench bottom under more than a thousand atmospheres of pressure. Because no single profile can capture every variation, the choice of model matters enormously when the answer is quoted to the nearest meter.

The preferred sound-speed model in the study combined two sources of oceanographic data. Upper-ocean observations from expendable probes, XCTD casts acquired by Hakuho-maru during the 2023 survey, extend to about 1,900 meters and capture surface conditions closest in time to the mapping campaign. Below that, the model draws on a full-depth CTD profile collected by the same vessel in 1992, providing sound-speed information through the entire water column. This pairing is notable because it links observations made more than thirty years apart, and it reflects the practical reality that modern surveys rarely have access to a contemporaneous full-depth profile at such extreme depths. Using this model, the team derived maximum gridded depths of 10,926 meters in the western basin, 10,912 meters in the central basin and 10,927 meters in the eastern basin, making 10,927 meters the study’s preferred estimate of the Challenger Deep’s maximum depth.

The authors are careful to note that this figure does not render other recent estimates wrong. Full-ocean-depth measurements from the 1984 Japanese survey vessel Takuyo, which yielded a commonly cited value of about 10,920 meters, through modern full-ocean-depth campaigns and the dives of the Five Deeps Expedition, each rest on their own vertical references, observing conditions and processing pipelines. The lesson of the sensitivity tests is that meter-scale comparisons between such numbers must state which sound-speed model, vertical datum and editing procedures were used. A depth of 10,927 meters and a depth of 10,914 meters can both be legitimate descriptions of the same seafloor under different, defensible assumptions about the ocean above it.

Another subtlety concerns what a single acoustic beam actually measures. The EM124’s beams have a finite width, configured at 2 degrees by 2 degrees on Hakuho-maru. Under a simple flat-seafloor approximation, at a depth of 11,000 meters each beam illuminates a footprint roughly 400 meters across in both the along-track and across-track directions. A quoted depth such as 10,927 meters is therefore not a pinpoint reading of a single point on the seabed. Each return represents an average over a finite patch of seafloor, and the final bathymetric surface is reconstructed from many overlapping soundings. Narrow depressions or sharp topographic relief may be smoothed or missed entirely when the footprint grows to hundreds of meters, which means the deepest point in a V-shaped trench cross-section could plausibly sit lower than any gridded value suggests.

Survey conditions proved to be a second major control on data quality. The main east-west survey lines were run at 4 knots, with additional data collected at 8 and 15 knots. After quality editing, about 87 percent of soundings were retained from the 4-knot east-west lines and 84 percent from the 8-knot lines, compared with 80 percent from the 4-knot north-south lines and 78 percent at 15 knots. At the fastest speed, the greater spacing between successive pings produced visibly sparser coverage of the seafloor, and even at identical speeds the north-south lines showed greater variability where the vessel crossed steep topographic gradients. Ship motion, line orientation and vessel speed, in other words, leave measurable fingerprints on the resulting map.

These findings carry a broader implication: precise mapping of the deep seafloor cannot be separated from careful observation of the ocean above it. The sound-speed structure of the water column, the motion and speed of the survey platform, the geometry of the sonar beams and the choices made during data processing all combine to determine the final number. For a feature as celebrated as the Challenger Deep, where popular accounts often quote depths to single meters, the study provides a sobering reminder that uncertainty at full-ocean depth is intrinsically larger than most headlines imply. It also gives researchers a concrete framework for evaluating any future claim about the deepest point of the world ocean.

In an unusually open move, the team has released the complete underpinnings of their analysis: raw EM124 data, processed soundings, bathymetric grids, the seawater sound-speed models, vessel and sensor configuration details, and the full processing workflow. This openness allows the 2023 observations to be reprocessed whenever better full-depth oceanographic information or more precise vertical references become available. The dataset can also be compared directly with pressure measurements from deep-submergence vehicles and with higher-resolution observations made close to the seafloor, offering independent checks on the acoustic result. The next step, the authors suggest, is to test the 10,927-meter estimate more rigorously by combining such independent observation types.

The approach also points toward future exploration of poorly charted waters. The same methods and open-data philosophy are relevant to mapping polar regions using platforms such as the Japanese Antarctic icebreaker Shirase and the future Arctic research vessel Mirai II, where ice, weather and limited survey time make every sounding count. Published in the journal Scientific Data, the study demonstrates that even a classic measurement, the depth of the ocean’s deepest point, can be advanced not by a bigger number but by a more transparent account of how the number was made. In deep-sea science, knowing the error bars is as newsworthy as knowing the depth itself.

Subject of Research: Sensitivity of full-ocean-depth multibeam bathymetric measurements of the Challenger Deep to sound-speed models and survey conditions

Article Title: How accurately can we measure the ocean’s deepest point?

Article References: How accurately can we measure the ocean’s deepest point?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Challenger Deep, Mariana Trench, bathymetry, multibeam echosounder, sound-speed model, EM124, R/V Hakuho-maru, ocean depth measurement, deep-sea mapping, Scientific Data, acoustic sounding, XCTD