Located or happening beneath the surface of the ocean; underwater.
English compound formed from 'under' (Old English 'under') + 'sea' (Old English 'sæ'). The combination became common in the 19th century as underwater exploration technology developed.
Before submarines were invented in the 1800s, 'undersea' was mostly a poetic or theoretical concept—now it describes an entire universe of exploration that covers 71% of Earth's surface and remains largely unmapped.
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