One reason the sea is ripe for robot warriors is because remote control under water is harder than it is in the air. While current drones navigate by GPS and relay signals through radio or satellite uplink, those signals don't travel as well through the sea. An autonomous submarine, instead, could operate for days underwater, returning to the surface to transmit the data it collected and receive new instructions. Boeing has already built and the U.S. Navy is looking to buy a similar, maybe identical, large autonomous submarine to scout the sea and report back. Rather than competing for bandwidth in a crowded sky, unmanned sea robots will work on their own, without a need for step-by-step handholding.