Submarine

ocean · vehicles · underwater

Master of the deep. A submarine is a watercraft that can operate completely underwater, traveling beneath the surface of the ocean.

Always a boat. No matter how huge they get, submarines are always called 'boats' rather than 'ships'!

Robot subs too. The word 'submarine' is sometimes used for underwater robots and remotely operated vehicles that explore where people can't go.

On its own. Submarines can run independently underwater, carrying everything they need to keep going beneath the sea.