It's hard to covertly gather intelligence on the open seas; conventional drones and submarines make it rather obvious that something's up. The US Navy might just have a stealthier option with its experimental GhostSwimmer drone, which recently finished tests. The five-foot-long robot looks and swims much like a shark, letting it spy on enemies (or inspect friendly ships) without being conspicuous -- as long as curious foes don't get too close, anyway. It goes as deep as 300 feet, and it can operate autonomously for long enough that the Navy doesn't have to stay nearby to keep watch.
There are catches to this biomimetic machine: You need to be on a 500-foot tether to maintain direct control, and the untethered GhostSwimmer still needs to surface to transmit its findings. And simply speaking, it's not ready for action yet. The Navy hasn't said if or when its fish-like recon tool will enter service. It's being developed at a research division focused on quick turnarounds, however, so it won't be surprising if this robotic predator is soon keeping an eye out for hostile boats and mines.
[Image credit: Mass Communication Specialist 3rd Class Edward Guttierrez III/US Navy]