Full-ocean-depth EM124 mapping of the Challenger Deep by R/V Hakuho-maru and depth-estimate sensitivity tests

The Challenger Deep in the Mariana Trench is the deepest known point on Earth, yet recent depth estimates still differ by several meters because of environmental and instrumental uncertainties. Here we present a new full-ocean-depth multibeam bathymetric dataset collected with a Kongsberg EM124 echosounder aboard R/V Hakuho-maru during the KH-23-9 cruise in 2023. The survey comprises east–west and north–south lines acquired at vessel speeds of 4–15 kt. Five seawater sound velocity models were constructed from contemporaneous XCTD data, historical full-depth CTD data, and a recent full-depth reference profile to test the sensitivity of depth estimates. After ping-by-ping editing and gridding, the preferred KH23XCTD+KH92CTD solution resolves three basins deeper than 10,900 m, with maximum depths of 10,926, 10,912, and 10,927 m in the western, central, and eastern basins, respectively. Alternative sound velocity models yield maximum depths from 10,914 to 10,932 m, illustrating the meter- to >10-m-scale sensitivity of full-ocean-depth multibeam estimates to the applied sound velocity structure. Cleaned point soundings, gridded bathymetric surfaces for each sound velocity model, and the gridding script are provided. The dataset enables assessment of how sound velocity structure, survey speed, and beam-footprint geometry influence depth estimates in full-ocean-depth multibeam mapping, and provides a new reference for cross-comparison of Challenger Deep bathymetry.

Exploration of the deepest regions of the Earth’s surface has been a longstanding scientific challenge spanning centuries. The Challenger Deep in the Mariana Trench (~11.3°N, 142.2°E) is recognized as the deepest part of the ocean floor. Acoustic bathymetric and pressure-based measurements in this region have a long and distinguished history, beginning with early soundings by the British ship HMS Challenger II1, followed by the Soviet ship Vityaz during the 1957 International Geophysical Year, and the American manned bathyscaph Trieste in 19602.

Since these pioneering efforts, modern multibeam surveys have progressively improved our understanding of the trench geometry. Japanese vessels have played a key role, including S/V Takuyo in 19843, R/V Hakuho-maru in 19924,5, and R/V Kairei in 1998, 1999, and 20026,7. Additional surveys by the American vessel USNS Sumner in 20108, German ship R/V Sonne in 20169, and DSSV Pressure Drop in 2019–202010,11 have further refined the bathymetric and depth-control framework of the Challenger Deep.

Recent studies have also highlighted the difficulty of assigning a single absolute depth to the deepest point. Gardner et al.8 reported a 95% confidence uncertainty of ±25 m for their multibeam-derived estimate, while van Haren et al.9 reported an uncertainty of ±12 m based on multibeam bathymetry supported by full-depth CTD observations. The DSSV Pressure Drop survey, conducted with a Kongsberg EM124 multibeam echo sounder operating at 8 kt, yielded a deepest value of 10,924 ± 15 m in the eastern basin when combined with full-depth CTD profiles collected by the submersible DSV Limiting Factor10. Concurrently, Greenaway et al.11 reported a revised depth of 10,935 m ± 6 m at 95% confidence, derived from the submersible altimeter profiles referenced to in-situ pressure. Notably, Greenaway et al.11 incorporated detailed corrections for seawater density structure, atmospheric pressure, water level, local gravity anomalies, and vertical gravity gradients in converting pressure to depth. Such corrections are essential because even small deviations from standard gravity can accumulate into several meters of systematic bias at ~11 km depth.

Thus, although the Challenger Deep remains Earth’s deepest known location, small but persistent discrepancies among recent depth estimates highlight the need to document how survey conditions and processing choices influence full-ocean depth multibeam echo-sounding. In this study, we present new multibeam sounding data collected with the newly installed Kongsberg EM124 system aboard R/V Hakuho-maru during cruise KH-23-9 on 30 November 2023. Using these observations, we test the sensitivity of gridded depth estimates to seawater sound velocity structure, vessel speed, and system operating mode. In particular, differences among the sound velocity structure models considered here change the estimated maximum gridded depth by up to 18 m. We also examine how beam-footprint geometry and gridding choices affect the representation of small-scale depressions at full-ocean depth.

We further show that the resulting bathymetry, including the three principal basins that each exceed 10,900 m, is consistent with previous surveys in its large-scale morphology. The preferred KH23XCTD+KH92CTD solution gives a maximum gridded EM124 depth of 10,927 m in the eastern basin, whereas alternative sound velocity models yield maximum depths from 10,914 to 10,932 m. We therefore present the 10,927 m value as the preferred gridded solution for this dataset, not as a definitive redetermination of the deepest point on Earth. This dataset provides a basis for cross-comparison of Challenger Deep bathymetry and for evaluating how sound velocity structure, survey configuration, and processing choices influence full-ocean-depth multibeam mapping.