How does pressure affect seawater density at great ocean depths?

At great ocean depths, seawater density increases due to the compressibility of water under extreme pressure. While seawater is often treated as incompressible in many contexts, the tremendous pressures found in the deep ocean cause the water to become denser as depth increases.1

This relationship is illustrated by the following data for seawater at 35 parts per thousand salinity and 0 °C:1

| Depth (m) | Pressure (decibars) | Density (g/cm³) |
| :--- | :--- | :--- |
| 0 | 0 | 1.02813 |
| 2,000 | 2,000 | 1.03747 |
| 4,000 | 4,000 | 1.04640 |
| 6,000 | 6,000 | 1.05495 |
| 8,000 | 8,000 | 1.06315 |
| 10,000 | 10,000 | 1.07104 |

If seawater were truly incompressible, the density would remain constant regardless of depth. Because it is compressible, the density values rise significantly as one descends, which has a measurable impact on global sea level; if the deep ocean were less compressed, the average sea level would be substantially higher.1