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The ocean floor covers more than 70 percent of Earth’s surface. Just as above-water landscapes are characterized by valleys, ridges, mountains, basins, and other topographic features, the seafloor surface is similarly diverse and constantly changing.

Whereas topographic features are often readily observed by scientists on land, the physical characteristics of the seafloor—collectively known as bathymetry—are less accessible, sometimes located miles beneath the surface where even sunlight cannot reach. To better understand these deep-sea bathymetric features, scientists rely on remote-sensing tools such as satellites, sonar, and submersible robots. 

Satellite-derived bathymetry of the seafloor—even miles-deep valleys such as Escanaba Trough—can be viewed on web-based applications such as Google Maps. Satellites measure deviations in sea surface height, created by gravitational attraction to seafloor features like ridges, trenches, and volcanoes. These maps offer widespread coverage of seafloor bathymetry, but at a relatively low resolution.

Research vessels improve bathymetric resolution by using multibeam sonar. These vessels emit a swath of sound pulses from the hull of the ship that are reflected off the seafloor. The timing of returning sound pulses is used to determine seafloor depth, while the intensity of the returning signal informs seafloor composition (e.g., solid rock or soft mud). This data is integrated into high-resolution seafloor maps. 

Animated GIF showing difference in seafloor mapping resolution from satellite and multibeam sonar
An animated GIF illustrating the difference in seafloor mapping resolution from satellite, multibeam sonar, and AUV data. Three bathymetric images from the same location in Escanaba Trough show 1) satellite-derived bathymetry at approximately 500-meter resolution (at this resolution, no seafloor features are visible and the image is blue); 2) multibeam sonar-derived bathymetry at approximately 100-meter resolution; and 3) AUV Sentry-derived bathymetry at approximately 1-meter resolution.

Sometimes, seafloor maps with even more detail are required. To better understand the composition and distribution of hydrothermal systems in Escanaba Trough and characterize its unique ecosystems, researchers used the autonomous underwater vehicle (AUV) Sentry and remotely operated vehicle (ROV) Jason to navigate to seafloor areas with known or suspected hydrothermal activity.

Two miles below the ocean’s surface in Escanaba Trough, the seafloor is spreading apart. New oceanic crust pushes up from Earth’s mantle only to be met with blankets of thick sediment, frigid temperatures, and complete darkness.  

Autonomous underwater vehicle Sentry being deployed from a research vessel
The autonomous underwater vehicle Sentry being deployed from the research vessel Thomas G. Thompson during the 2022 Escanaba Trough Expedition.

Cold seawater containing dissolved gases and minerals soaks through the sediment, squeezing into every crack, crevice, and pore of the new crust. Met with high temperatures, this seawater is transformed into hydrothermal fluid that rises back up through the seafloor, creating massive mounds and towering chimneys of metal-rich sulfides. These seafloor features were among the targets researchers mapped with the AUV Sentry.

After launching from the ship and descending to the seafloor, Sentry swam 70 meters from the bottom, following a route programmed by technicians onboard. Using multibeam sonar and an array of other sensors, Sentry data was used to generate seafloor maps with a resolution twenty times greater than the ship’s multibeam maps. 

Mapping closer to the seafloor with Sentry is like using a telephoto lens to take a picture. Zoom out and you’ll capture rolling hills and valleys. Zoom in and you’ll see those hills and valleys are full of mysterious pockmarks and clusters of narrow chimneys.

The interdisciplinary team of engineers, geophysicists, volcanologists, and geologists aboard the Escanaba Trough expedition used maps created by Sentry to plan dive locations for the ROV Jason. Dive locations included target features where samples were collected to help answer key scientific questions, as well as unidentified features that sometimes took scientists by surprise.

Aided by Sentry’s detailed maps and Jason’s powerful lights and high-definition video cameras, researchers could “see” the seafloor in astonishing detail, illuminating a part of this planet never before observed by humans.

The control room of the remote-operated vehicle Jason during the Escanaba Trough expedition
A look at the control room of the remote-operated vehicle Jason during one of its dives on the Escanaba Trough expedition.

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