Although still in the development stages, the technology has matured in recent years to be able to overcome the vast difficulties of operating underwater, a far more harsh environment than what aerial drones face in the sky.

Saltwater corrodes metal. Water pressure can be crushing at great depths. And communication is severely limited, so the vehicles must be able to navigate on their own without being remotely piloted.

Despite the immense difficulties, the Navy has been testing and fielding several new systems designed to map the ocean floor, seek out mines, search for submarines and even launch attacks. While the unmanned crafts are now able to stay out for days or weeks, the goal is to create an underwater network of service stations that would allow the vehicles to do their jobs for months — and eventually years.