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 an undersea fleet. undersea. adverb. variants: or underseas \ ˌən-​dər-​ˈsēz \

Is underseas a word?

beneath the surface of the sea.

What is meant by seafloor?

: the floor of a sea or ocean : seabed The Earth's crust, in this view, is divided into several immense plates that make up the continents and seafloors, and that all float on a hot, plastic, subterranean "mantle."— Walter Sullivan.