Bathymetry is the measurement of the depth and shape of the floor of the ocean, lakes and rivers. A bathymetric map does for the underwater world what a topographic map does for dry land.
Key takeaways
- Bathymetry means the depths and shapes of underwater terrain, usually drawn with colour or depth contours.
- This map is built on the GEBCO_2026 bathymetry grid, which gives one depth value every 15 arc-seconds.
- The deepest point in that grid is Challenger Deep, 10,927 m below sea level.
- A bathymetric map shows the seabed, not the sea surface, so it is a different thing from a nautical chart used for navigation.
How bathymetry works
The word comes from the Greek for "deep" and "measure". Originally it meant the depth of the ocean relative to sea level. It has since come to mean "submarine topography", the depths and shapes of underwater terrain [1].
A bathymetric map is the underwater equivalent of a topographic map. A topographic map shows the three-dimensional shape of the land. A bathymetric map shows the land that lies underwater [1]. The shape of the seabed is usually drawn in one of two ways: with colour, where each shade stands for a range of depths, or with lines called depth contours (also called isobaths), which join points of equal depth [1].
Bathymetry is also the foundation of hydrography, the wider science of measuring a body of water. Hydrography adds the shape of the shoreline, tides, currents and waves, and the physical and chemical properties of the water itself [1]. Bathymetry is the part that answers a simple question: how deep is it here, and what is the bottom like?
Modern bathymetry usually ends up as a grid, a regular pattern of cells with one depth value in each. The grid behind this map is the GEBCO_2026 release from GEBCO, with one value every 15 arc-seconds [2]. Ship surveys fill in the detail where they exist and satellite estimates fill the gaps, which is why some parts of the map are sharp and others blurry. The homepage explains that blend, and the glossary pages on sonar and satellite-derived bathymetry go further into each method.
An example from our data
Take the deepest place in the grid. Challenger Deep reads 10,927 m below sea level, and the single deepest cell in the whole grid, in the Mariana Trench, reads 10,931 m. Those two figures differ because they come from different spots in the same bathymetric grid.
Bathymetry also tells you what a whole ocean is like. The Pacific Ocean has a mean depth of 4,315 m, while the North Sea averages only 78 m. Bathymetry is what lets the slider work. When the sea is lowered by 120 m, 84.5% of it becomes dry on this simple model. The drain-the-oceans section explains the caveats, because the slider lowers the sea everywhere by the same amount on today's seabed and ignores ice sheets and changes to the land since.
The thing people get wrong
Many people assume that someone has surveyed the whole seabed in detail, like a road atlas. They have not. Satellites have covered the entire seafloor, but they give only a general picture, with limited detail, and some features such as seamounts and objects such as shipwrecks stay unseen [3]. NOAA reports that as of April 2026 only 28.7% of the global seafloor had been mapped with modern high-resolution multibeam sonar [3].
So a bathymetric map, including this one, mixes real measurements with estimates. Treat the broad shapes as reliable and the fine detail of remote areas as a guide.
Bathymetry vs nautical chart
| Bathymetry | Nautical chart | |
|---|---|---|
| What it is | The depth and shape of the seabed | A map made for ships, with hazards, marks and soundings |
| Used for | Science, learning, planning surveys | Safe navigation |
| This map | Yes | No: not for navigation |
Frequently asked questions
What is the difference between bathymetry and topography?
Topography is the shape of the land surface above water. Bathymetry is the same idea for ground that is under water. Many modern datasets, including the one behind this map, join the two into one grid.
How is bathymetry measured?
Mostly with sound from ships (sonar) and with satellites that measure the height of the sea surface. Our page on how the seabed is mapped covers both.
What does bathymetry show?
Depth below sea level and the shape of the seabed: trenches, ridges, plains, slopes and undersea mountains.
Sources
- What is bathymetry?, NOAA Ocean Service.
- GEBCO_2026 Grid, GEBCO.
- How much of the ocean has been explored?, NOAA Ocean Service.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).