undersea I. \| ̷ ̷ ̷ ̷| ̷ ̷\adjective Etymology:under (II) + sea, n. 1.: being or carried on under the sea or under the surface of the sea < undersea oil deposits > < undersea fighting > 2.: designed for use under the surface of the sea < undersea fleet > < undersea cable > II. adverb orunderseas\| ̷ ̷ ̷ ̷| ̷ ̷\ Etymology:under (II) + sea, n. or seas, plural of sea, n. : under the sea : beneath the surface of the sea < photographs taken undersea >