Submarines operate below the surface of the seas.

Solution (By Examveda Team)

One word substitute is Subterranean.

Superfluous: unnecessary, especially through being more than enough.
Surreptitious: kept secret, especially because it would not be approved of.
Perspicacious: having a ready insight into and understanding of things.
Subterranean: existing, occurring, or done under the earth's surface.