Earth & Space · Oceans

Mapping the Ocean Floor with Gravity

Most of the ocean floor has never been seen by a ship. But every undersea mountain tugs on the water above it and raises a tiny bump on the sea surface. A satellite called SWOT measures those bumps so precisely that scientists can map the seafloor from space. The maps on this page are our own, drawn in your browser from the published data.

Explore the gravity map

land and ice shelves

Featured regions, in full detail:

Elsewhere the map shows a coarser world view, with land and ice shelves in tan. The dashed boxes mark the featured regions.

Or try a ready-made profile:

Why most of the seafloor is still unmapped

Light fades out within a few hundred meters of seawater, and radar can’t get through the water at all. So a satellite can’t photograph the bottom of the deep ocean. To measure depth, a ship sends pulses of sound straight down and times the echoes. This is sonar. It works well, but a ship maps only a narrow strip as it goes, and the ocean is huge. Ships with sonar have mapped only about a quarter of the seafloor in detail.

For the rest, scientists use a trick: they let gravity do the looking.

How gravity shows the seafloor

Rock is almost three times as dense as seawater. An undersea mountain, called a seamount, adds extra mass, and that mass pulls a little harder on the water around it. Water piles up over the seamount, so the sea surface stands a few centimeters higher there. Over a deep trench, the surface dips.

Average out the waves, tides and currents, and the shape of the sea surface becomes a map of gravity. The map on this page shows the vertical gravity gradient (VGG). It tells how quickly gravity changes as you go up or down. It shows edges and small features very sharply, which makes it the best gravity map for spotting seamounts, ridges and hills.

1,000 m

  1. Mass pulls water. Rock is denser than water, so a seamount adds mass. Its gravity pulls the nearby water toward it, and the sea surface bulges up over it, by a few centimeters for a seamount a kilometer tall.
  2. The satellite measures the surface. SWOT's two antennas, 10 meters apart, catch the same radar echoes. Comparing them gives the height of the sea along two strips about 50 km wide, one on each side of its track, to within about a centimeter. Over many passes, the bumps add up into a map.
  3. Slope and curvature give gravity. Where the sea surface tilts, gravity is pulling sideways. The tilt in every direction gives the gravity anomaly. How sharply the surface curves gives the gravity gradient: it equals normal gravity times that curvature, so a small, sharp bump shows up clearly.
  4. Eötvös, the unit. One Eötvös (1 E) is 10⁻⁹ per second squared: gravity changing by one billionth of a meter per second squared for every meter you move up or down. Change the height above and watch the peak: it grows fast with size.
A simplified model, not a survey: the seamount is a cone four times as wide as it is tall, treated as one lump of extra mass (rock 2,800 kg/m³, seawater 1,030 kg/m³) in water 4,000 m deep. Real seamounts are often partly supported by the bending plate beneath them and buried in sediment, which makes their signals weaker.

What SWOT is

SWOT stands for Surface Water and Ocean Topography. It is a satellite mission of NASA and CNES, the French space agency, with help from the Canadian Space Agency and the UK Space Agency. It launched in December 2022. Its main job is to measure the height of water in oceans, lakes and rivers.

Older satellites measured sea height along a thin line straight below them. SWOT’s main instrument, KaRIn (the Ka-band Radar Interferometer), has two radar antennas 10 meters apart, at the ends of a mast. The radar echo from each spot on the sea reaches the two antennas along slightly different paths. Comparing the two signals (this is called interferometry) gives the height of the sea surface across a wide swath: a strip about 50 kilometers wide on each side of the satellite’s track, with a narrow gap right below it. SWOT covers about 90 percent of the globe every 21 days and measures height to within about a centimeter.

Because it sees a whole swath at once, SWOT measures the slope of the sea surface in every direction, not just along one line. In 2024, Yao Yu, David Sandwell and Gérald Dibarboure showed that one year of SWOT data reveals more detail in marine gravity than 30 years of the older satellites, down to features about 8 kilometers across.

Before and after. Drag the slider. Left: V33, the older map, made from 30 years of satellites that measure only straight down. Right: SWOT, version 06. Same place, same scale, same colors. Look for the small seamounts and the grain of the abyssal hills.

What the new maps reveal

  • Many more seamounts. Earlier satellites could find seamounts taller than about a kilometer. SWOT can find ones less than half that tall. The number of known seamounts could grow from about 44,000 to about 100,000. Seamounts matter to life: currents bring nutrients up their slopes, and fish and corals gather there.
  • Abyssal hills. These are the most common landform on the ocean floor. They cover about 70 percent of the ocean floor. Each is only a few kilometers wide, and they form in long parallel bands where plates spread apart. Before SWOT, they had been mapped only by ships. Now they can be traced across whole oceans.
  • The history of the plates. The direction of abyssal hills and fracture zones records how plates moved over millions of years. SWOT also shows structures buried under sediment on continental margins and old spreading ridges in ice-covered seas.

What the maps are good for

  • Navigation. Ships and submarines use seafloor maps, and so do the crews who lay communication cables on the bottom.
  • Ocean mixing. Tides and deep currents flowing over rough seafloor stir the deep ocean. That mixing moves heat and nutrients around. Researchers still don’t fully understand how abyssal hills affect it, and better maps help.
  • Plate tectonics. The new maps let geologists reconstruct how the ocean basins opened, piece by piece.

What gravity maps can’t tell you

A gravity map shows where a feature is and its shape. It does not measure depth directly. To turn gravity into depth, scientists must assume how dense the rock is and how much soft sediment covers it, and those guesses can be off. Gravity also blurs: the deeper a feature lies, the weaker and wider its pull at the surface, so the smallest features are lost. For exact depths, and for the charts ships rely on, sonar from ships is still needed. The best maps combine both: gravity fills the gaps between ship tracks, and sonar pins down the depths.

Reading the map

  • Bright (or red, on the diverging scale) means gravity is stronger than average and grows fast as you go down: extra mass close by, such as a seamount or ridge.
  • Dark (or blue) means missing mass, such as a trench or a valley.
  • A ring of dark around a bright seamount is normal. It comes from the way gravity’s gradient changes sign a short way off to the side.
  • Tan areas are land or ice shelves, which this map leaves out.
  • The units are Eötvös (E), named after the Hungarian physicist Loránd Eötvös. One Eötvös is 10⁻⁹ per second squared. In everyday terms: if gravity grew stronger by one billionth of a meter per second squared for every meter you went down, that would be 1 E. Earth’s normal gravity changes by about 3,086 E. The map shows only the small differences on top of that, usually tens of E and up to a few hundred over big seamounts.
  • The gravity anomaly layer, in milligals (mGal), shows gravity itself rather than its gradient. It is smoother, and good for big features. One milligal is a hundred-thousandth of a meter per second squared, about a millionth of normal gravity.

Watch NASA’s video

NASA’s Scientific Visualization Studio made a striking flyover of the SWOT gravity map: Surface Water and Ocean Topography (SWOT) Vertical Gravity Gradient, visualized by Greg Shirah with scientists David Sandwell and Yao Yu. It inspired this page. We link to it rather than copy it. The maps here are our own renderings of the published data.

About these maps

The maps, profiles and animation on this page are our own renderings, made from the published gravity grids. We cut the grids into small tiles that store the data values, not pictures, and your browser colors them. The world view is coarse, and eight featured regions have full detail. The values you see when you hover have been rounded slightly, by less than 1 E for small values.

  • SWOT gravity, version SWOT_06 (September 2026): vertical gravity gradient and free-air gravity anomaly, from the Scripps Institution of Oceanography data folder. It uses three years of SWOT data.
  • The older map, V33: the IGPP vertical gravity gradient, version 33, from nadir altimeters only, as served by the Generic Mapping Tools.
  • Land and ice shelves: Natural Earth.

If you use these data, please cite the publications the data’s authors ask for:

  • Yu, Y., Sandwell, D. T., & Dibarboure, G. (2024). Abyssal marine tectonics from the SWOT mission. Science, 386(6727), 1251–1256. doi:10.1126/science.ads4472
  • Sandwell, D. T., Harper, H., Tozer, B., & Smith, W. H. (2021). Gravity field recovery from geodetic altimeter missions. Advances in Space Research, 68(2), 1059–1072. doi:10.1016/j.asr.2019.09.011
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