One of the largest unmanned submarines ever built is finally performing sea trials. But don’t expect the U.S. Navy, which dreams of undersea drones that can span oceans, to proclaim the Proteus its drone sub of the future. Instead, Proteus’ manufacturers want to work with the Navy to test the software, sensors and power systems that will define those next-gen drone subs — and maybe use the Proteus as a stopgap solution until someone develops those long-range submarines.

We first ran across the beastly Proteus at a special-operations industry conference in May 2011. It was hard to ignore: a 25-foot, 6,200 lb. black cylinder hauling two 220-lb. bomblets, with room to fit 400 lbs. of submerged cargo, or six Navy SEALs in its optional manned mode. By contrast, the SeaFox, an undersea drone the Navy is using to spot Iranian mines, is a paltry four feet long.