'deep-sea' originates from the combination of 'deep' and 'sea', where 'deep' meant 'extending far down from the top or surface' and 'sea' referred to 'a large body of saltwater'.

The term 'deep-sea' has been used since the 19th century to describe the parts of the ocean that are far from land and below the surface.

Initially, it referred to any part of the sea that was not near the shore, but over time it has come to specifically mean the deeper parts of the ocean.