Sweden has developed a modular torpedo (AUV62) that operates as a UUV (unmanned underwater vehicle.) Because the 21 inch (53cm) torpedo is modular, it weighs .7-1.5 ton and is 3-10 meters (9.3-31 feet) long, depending on what modules are used. It uses new, lighter and more powerful sensors to take pictures (even when no light is available, using Synthetic Aperture Radar, or SAR) or do sonar mapping. AUV62 is most useful looking for bottom mines, and can scan two square kilometers of sea floor per hour. AUV62 can also be equipped with transmitters that make it appear, to other submarines or surface ships, as a submarine. This is used for training, or as a deception device in wartime.
AUV62 communicates via an acoustic (sonar) link that has a range of 3-15 kilometers. When it pokes its short mast above the surface, it can communicate via radio link (up to 8 kilometers, or anywhere if satellite datacomm is used). Endurance is about six hours, depending on speed. AUV62 can move as fast as 36 kilometers an hour, but usually moves along at less than 10 kilometers an hour when mapping or searching.
AUV62, and similar devices (that won't fit into a torpedo tube) have been under development since the 1990s in Sweden. As these UUVs were increasingly used, over the last few years, for testing and training, much more detail, about what's on the Baltic seabed, has been discovered. These include World War II torpedoes, that failed to explode, shipwrecks, and lots of old naval mines. During the two World Wars, over 100,000 such mines were used the Baltic. Only about 40,000 of them were found and removed after the wars. Nearly all of these mines were of the contact type (the sphere with the "antennae" coming out of them). These are kept in place by a chain attached to an anchor on the sea floor. After a while, these mines sink to the sea bottom, where they eventually decay to the point where they are no longer dangerous. With search devices like AUV62, it's now possible to map the location of many of these "lost" mines.