These days, data centers, portable USB battery chargers and all sorts of transportation are going green thanks to fuel cell technology. Now you can add submersibles to that list: Japan is developing a 33-foot-long submarine that runs off emissions-free energy. Japanese Defense Ministry officials told AFP that the finalized tech is about five years out, but they estimate the $25 million unmanned sub (or UUV) will be capable of cruising deep waters for an entire month on a single charge. Sure, 30 days seems pale in comparison to the 50-year core life of the nuclear-powered USS Nautilus, but electro-chemical energy is far cleaner, and can be much cheaper in the long run. This driverless U-boat also won't have any artillery on board. According to a ministry official, "The UUV is purely for patrolling -- it's not a military weapon," so there's no need to worry about underwater combat drones -- at least, for now.