Depth is the vertical distance from the ocean surface down to a given point, most often the seafloor. It is a fundamental coordinate in oceanography, controlling pressure, light penetration, temperature, and the types of life that can exist.

Average ocean depth is about 3,700 meters; the deepest trenches exceed 10,000 meters. Depth is measured by echosounders that time the round trip of an acoustic pulse, by pressure sensors on instruments, or by satellite altimetry that infers seafloor topography from surface height. Accurate depth information is required for navigation, cable routing, habitat classification, and the interpretation of every water-column profile.

Because pressure increases by roughly one atmosphere every ten meters, depth also defines the physiological limits for divers and the engineering requirements for submersibles and sensors. Many biological and chemical processes are organized by depth zones—euphotic, mesopelagic, bathypelagic—each with characteristic conditions.

• Vertical distance from surface to a point in the ocean
• Controls pressure, light, and habitat
• Measured by sonar, pressure, or satellite methods
• Average ocean depth about 3,700 m