noun, noun which may take the genitive case particle 'no'
below sea level; undersea; beneath the waves
Refers to the area or state of being under the surface of the sea. Often used in scientific or descriptive contexts.
その島の一部は海面下にある。
Part of the island is below sea level.
海面下の地形を調査する。
Survey the undersea topography.
海底 refers to the seabed or ocean floor, whereas 海面下 describes the region beneath the sea surface, not necessarily the bottom.
Compound of 海面 (sea surface) and 下 (below, under).