Drones require infrastructure to function. You've got to launch them from somewhere, and if you want them back, you have to land them somewhere, too. And infrastructure, as a general rule, is not secretive or stealthy, which can cause problems for the military, since they like being stealthy. As far as the U.S. Navy goes, nothing is stealthier than a submarine, so turning one of those into a mobile drone launcher like the Naval Research Laboratory (NRL) just did makes perfect sense.
Submarines are already equipped with ways of sending stuff to the surface from depth. By "stuff," we're generally talking about missiles, ranging from Tomahawks all the way up to ballistic missiles. So all you really have to do to launch a drone from underwater is replace one of those Tomahawks with a drone in a tube, fire it off, and then instead of engaging some sort of rocket motor, let the tube surface and then gently send the drone on its way. Easy!
The drone that the NRL used for this test was an XFC UAS (that would be, "eXperimental Fuel Cell Unmanned Aerial System"), fired from a Los Angeles class (i.e. very big, nuclear powered) submarine from some unspecified depth. Once the launch tube hit the surface, it stabilized floating upright, and an electric launch assist system helped the drone get airborne. Six hours later, the drone landed at the Naval Sea Systems Command Atlantic Undersea Test and Evaluation Center in the Bahamas.
It took just six years to take this idea from a concept to this demonstration, which is apparently not a very long time as far as the Navy is concerned. And that's great. But, we're kind of wishing that the Navy had instead developed something like the much, much cooler Cormorant drone from Lockheed Martin. You know, this thing:
That was supposed to work like this:
And actually got as far as this:
It's just a mockup, but that's a reasonably significant step for a DARPA-funded concept to get to. Cormorant was sadly canceled in 2008 due to budget cuts, but we still think it's one of the most innovative (and coolest looking) submarine/drone hybrids we've ever seen.