The threat of terrorists attacking ships (military or civilian) via underwater swimmers (using scuba gear, or a small submarine) has led to the demand for defenses. The U.S. Navy has trained dolphins, and other sea mammals, for that kind of work. But now there�s the Cerberus360 swimmer detection system. This is a (large) refrigerator size device that is lowered to the ocean floor in the middle of the area you want to guard. Cerberus360 uses sonar to detect anything, large enough to be a threat, up to 800 meters out. Actually, during tests, it was able to detect an approaching underwater scuba swimmer at 900 meters. Cerberus360 works well in shallow water, and can be tweaked by the operator, once emplaced, to be even more accurate. Al Qaeda groups are know to have bought scuba gear and trained for attacks like this, but none have been attempted yet.