海底 literally means 'sea bottom' — 海 (hǎi) means 'sea' or 'ocean', and 底 (dǐ) means 'bottom' or 'base'. Together, they form a compound noun referring specifically to the ocean floor: the solid surface beneath seawater, including sediment, rock, and underwater geological features. It’s a neutral, factual term commonly used in geography, marine biology, and environmental science.

This word appears frequently in contexts describing underwater exploration, marine habitats, or geological processes — for example, when discussing hydrothermal vents, coral reefs, or submarine cables. Unlike more poetic or literary terms like 深海 (shēn hǎi, 'deep sea'), 海底 emphasizes physical location rather than depth or mystery. It’s often paired with verbs like 位于 (wèiyú, 'to be located at'), 探测 (tàncè, 'to survey'), or 覆盖 (fùgài, 'to cover').