[Research Organization of Information and Systems]. How deep is the deepest ocean—and how precisely can we actually measure it?
Bathymetric maps obtained by each sounding at various survey speeds and modes. Each panel shows depth data distribution obtained along survey and transit lines; (a) east to west survey lines at 4 kt in electric thrust mode, (b) north to south survey lines at 4 kt in electric thrust mode, (c) transit lines at 8 kt in electric thrust mode, (d) transit lines at 15 kt in direct mechanical thrust mode. The contour interval is 20 m. The yellow contour shows water depth of 10,900 m.
Editor’s note: when we begin to robotically explore the ice-covered oceans of Enceladus, Europa, Ganymede, Mimas etc. we’ll need to make the most of the visits. Learning how to best operate robotics systems in a deep ocen environment searching for unknown and unexpected life forms is something we can perfect – now – on Earth.
A Japanese research team led by the National Institute of Polar Research (NIPR), SOKENDAI, the University of Tokyo, and other Japanese institutions has mapped the Challenger Deep in the Mariana Trench using the EM124 multibeam echosounder aboard the research vessel Hakuho-maru.
The Challenger Deep has been measured repeatedly for decades, from the 10,920 m depth value based on the 1984 survey by the Japanese survey vessel Takuyo to recent full-ocean-depth surveys and dives including the Five Deeps Expedition. Yet even modern high-precision measurements still differ by several to more than 10 meters.
Rather than seeking a new “deepest point” record, the study asks a more fundamental question: what controls a depth estimate when the seafloor lies nearly 11,000 meters below the ocean surface?
An 18-meter spread—and a preferred estimate of 10,927 m
Acoustic depth measurement is based on a simple principle: depth ≈ sound speed in seawater × round-trip travel time ÷ 2.
At the Challenger Deep, sound takes roughly 15 seconds to travel from the ship to the seafloor and back. But sound speed in seawater varies with temperature, salinity, and pressure.
Bathymetric observation at the Challenger Deep during R/V Hakuho-maru cruise of KH-23-9. (a) survey area (red box) shown with trench geometry in plate subduction system surrounding Philippine Sea Plate. (b) overall view of R/V Hakuho-maru. (c) EM124 installed on the bottom of R/V Hakuho-maru. (d) instrument configuration of transducer, motion reference unit (MRU), and GNSS sensor.— Scientific Data
The team processed the same multibeam dataset using five different seawater sound-speed models. The maximum gridded depth in the eastern basin ranged from 10,914 to 10,932 m, a spread of 18 m caused by the choice of sound-speed model.
The preferred model combined upper-ocean XCTD observations acquired by Hakuho-maru in 2023, extending to about 1,900 m, with a full-depth CTD profile acquired by the same vessel in 1992. This combination reflects upper-ocean conditions closest in time to the 2023 survey while also providing sound-speed information through the full water column.
Using this model, the maximum depths of the western, central, and eastern basins were 10,926 m, 10,912 m, and 10,927 m, respectively. The study therefore gives 10,927 m as its preferred estimate of the maximum depth of the Challenger Deep.
The result does not mean that other recent depth estimates are simply wrong. Rather, it shows that meter-scale comparisons at full-ocean depth must consider the sound-speed model, vertical reference, observing conditions, and processing method used to derive the value.
At 11,000 m, a beam does not measure a single pinpoint
Each acoustic beam emitted by a multibeam echosounder has a finite width. At 11,000 m depth, the 2° × 2° beam configuration of Hakuho-maru’s EM124 corresponds, under a simple flat-seafloor approximation, to a footprint roughly 400 m across in both along-track and across-track directions.
A depth such as 10,927 m therefore does not represent a single pinpoint on the seafloor. Acoustic returns represent a finite area, and many soundings are combined to reconstruct the bathymetric surface. Narrow depressions or sharp relief may not be fully represented when the footprint becomes large.
How the ship moves is also part of the measurement
The quality of the bathymetry also depended on survey conditions.
The main east–west lines were acquired at 4 knots, while additional data were obtained at 8 and 15 knots. After data editing, about 87% of soundings were retained for the 4-knot east–west lines and 84% at 8 knots, compared with 80%for the 4-knot north–south lines and 78% at 15 knots.
At 15 knots, the larger spacing between successive soundings produced sparser coverage. Even at the same speed, the north–south lines showed greater variability where the ship crossed rapid changes in seafloor topography.
These results show that precise full-ocean-depth mapping depends not only on the echosounder itself, but also on vessel speed, survey-line orientation, ship motion, sound speed, and beam footprint.
Open data for future reanalysis
The study makes publicly available the raw EM124 data, processed soundings, bathymetric grids, seawater sound-speed models, vessel and sensor configuration information, and processing workflow.
This allows the 2023 observations to be reprocessed if better full-depth oceanographic information or more precise vertical references become available. The dataset can also be compared with pressure measurements from deep-submergence vehicles and higher-resolution observations made closer to the seafloor.
The study also highlights the long-term value of ship-based observations: the preferred model links Hakuho-maru data collected more than 30 years apart.
The next step is to test the 10,927 m estimate more rigorously by combining independent observations. More broadly, the results show that precise mapping of the seafloor cannot be separated from observing the ocean above it.
This approach is also relevant to future exploration of poorly observed polar regions using platforms such as the Antarctic icebreaker Shirase and the future Arctic research vessel Mirai II.
Full-ocean-depth EM124 mapping of the Challenger Deep by R/V Hakuho-maru and depth-estimate sensitivity tests, Scientific Data (open access)
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