Exactly the same factors that are driving the rise of swarms of aerial drones are also leading to underwater drone swarms: fleets of small, low-cost submarines capable of networking to carry out missions which were traditionally limited to much larger manned vehicles.
In particular, over the past twenty years a new type of unmanned underwater vehicle (UUV) has been transforming underwater research. Unlike other UUVs which are typically tethered and have a very short range, underwater gliders can roam over long distances for months at a time. They are typically a couple of metres long, weigh about fifty kilos, and look like small torpedoes with wings.
Rather than propellers, gliders have a buoyancy engine. This pumps a small quantity of oil from an external bladder to an internal one, changing the density of the glider so it starts to descend. As it falls, it glides on a shallow trajectory, reaching a speed of about half a knot. After falling several hundred metres, and travelling a few kilometres, the glider pumps the oil the other way and starts to ascend, gliding upwards at the same leisurely speed. It is a slow but frugal form of travel with a tiny power requirement. In 2009 the Scarlet Knight glider operated by Rutgers University completed an Atlantic crossing in seven months on one battery charge.
Such gliders are now being developed for anti-submarine warfare, and this may have major implications for new submarine programs. Read my report in New Scientist here.