Below sea level means lower than the zero height used for the ocean's surface. Most of it is underwater, which is the seabed, but a few places on land also lie below the line, such as some desert basins and low coastal areas.
Key takeaways
- Anywhere with a height under zero is below sea level, whether it is the seabed or a basin on land.
- On our grid 71% of the world's surface is below sea level, which is 362 million km².
- The deepest cell is 10,931 m below the line.
- Land below sea level, such as Death Valley, stays land on the drain model.
How "below sea level" works
Heights are measured from the sea level reference, so anything lower has a negative value. In a bathymetry grid this is how the whole seabed is stored: each cell holds a number, and cells under the ocean are negative [1]. The deeper the seabed, the more negative.
Most of what is below sea level is under water. The shelf, slope and deep ocean floor are all below it, from a few metres off a beach to the bottom of the deepest trench.
A few places on land are below sea level too. They are usually basins cut off from the sea, such as parts of deserts and some low coastal plains. Water does not flood them for one of two reasons: a barrier such as higher ground or a sea wall holds the sea back, or the basin is simply too far inland.
So "below sea level" describes a height, not whether a place is wet or dry. It is also measured against an average, because the sea surface moves with tides and weather [2].
An example from our data
On the GEBCO_2026 grid, 71% of the world's surface is below sea level, which is 362 million km². Above it lies 147.9 million km².
The Mariana Trench region has the deepest cell, 10,931 m below the line. In contrast, the highest cell reaches 8,627 m above it. The gap between the highest ground and the deepest point is a reminder of how far the seabed falls.
Now try the drain slider. As the sea is lowered, shallow seabed rises above the new line and becomes land. At 120 m lower, 21.7 million km² is exposed. But basins on land that are below today's zero stay land in this model, because the slider only lowers the sea over the seabed. Death Valley is an example.
The thing people get wrong
People often assume "below sea level" means flooded or underwater. It does not. Land can sit below the zero line and still be dry, because water only reaches it if there is a route. The seabed is below sea level and is always wet, but those are different cases of the same word.
The drain model also does not flood basins on land. It lowers the sea; it never pours water into low ground.
Sources
- GEBCO_2026 Grid, GEBCO.
- Is sea level rising?, 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).