The British Royal Navy has adopted a version of the U.S. Remus 600 UUVs (Unmanned Underwater Vehicle) for underwater reconnaissance (including searching for mines). The U.S. Navy earlier used the similar, but smaller (80 pound), Remus 100s for this in Iraq, and the Remus design is highly regarded.
Remus 600 is an 240 kg (528 pound) pound vehicle that looks like a small torpedo. It is 3.25 meters (10 feet) long and 320mm in diameter. Carrying a side scanning sonar, and other sensors, a Remus 600 can stay under water for more than 24 hours, traveling at a cruising speed of 5.4 kilometers an our (top speed is nearly twice that.) The UUV can operate up to 100 kilometers from its operator, and dive to 600 meters (1900 feet). The UUV keeps costs down by using GPS, in addition to inertial guidance. The UUV surfaces every hour or two to get a GPS fix, and then goes back to doing what it was programmed to do.
Remus 100 was designed mainly for civilian applications (inspecting underwater facilities, pollution monitoring, underwater survey or search). But there were similar military and police applications, like searching for mines, or other terrorist activities. Australia and New Zealand also used Remus 100, and over a hundred are in use. This success led to the development of the larger Remus 600. Depending on sensors carried, each Remus 600 costs $500,000-1,000,000.