UUVs (unmanned underwater vehicles) have tended to look like torpedoes, if only because that makes it possible to launch them from the torpedo tube of a submerged submarine. But there are some interesting exception. One is the Theseus. Built in the 1990s for the U.S. and Canadian navies, to lay cable under the arctic ice, it is 50 inches (1.27 meter) in diameter, 11 meters long and weighs about nine tons. The Thesus can be out and about for over 60 hours on one mission. Much of its military use is classified, but it, and similar, "large UUVs" are apparently very popular for espionage operations.
More typical of the torpedoes easily mistaken for a torpedo is the Swedish modular torpedo (AUV62) that operates as a UUV. 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. Larger UUVs like the Thesus carry the same sensors, although the greater interior capacity allows for more powerful versions of these sensors.
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.
The technology developed for the smaller UUVs, works just as well for the larger ones. But the big UUVs are not as cost-effective for civilian operations. Most of the big UUVs are for military missions, and usually ones that are highly classified.