The new frontier for drone warfare is underwater, from tiny 7kg microsubs to the 15-metre Echo Voyager
The Pentagon envisages a future undersea battlescape in which unmanned combatants spend years patrolling offshore at a time
As unmanned aerial drones have become a critical part of modern warfare, the Pentagon is now looking to deploy autonomous robots underwater, patrolling the sea floor on a network complete with rest stops where the drones could recharge.
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 craft 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.