December 3, 2007: In the U.S., the Navy and Coast Guard have developed hand grenade sized "depth charges" for use against hostile scuba divers out to plant bombs on ships in harbors. The war on terror has made navies nervous about this danger from scuba divers. Terrorists swimming underwater can attach explosives to the side of a ship, which will punch a large hole in the hull. This happened several times during World War II (less than a decade after scuba gear was invented).

Even before September 11, 2001, there were indications that Islamic terrorists were interested in carrying out these kinds of underwater attacks. Despite the threat, there aren't too many ways to attack the swimmers. Bullets from rifles and machine-guns tend to break up on hitting the water. A swimmer deeper than three feet is generally immune to gunfire. The other popular weapon is the concussion grenade. These have been available since World War II, and are meant to be used to clear out bunkers or rooms with blast, not fragments. So they have twice as much explosives (half a pound) as normal grenades, and are largely made of cardboard. As an underwater weapon, they create a shock wave in the water that can kill or disable a diver up to about ten feet away. The problem is that these grenades have a 4-5 second delay, so they don't usually sink more than ten feet before exploding. Another option is special bullets for heavy machine-guns, that will not fragment when they hit the water. These can be effective down as deep as 30 feet. You can see where this is going. If a diver swims at 35 feet, he is only vulnerable to other divers, or the small number of "attack dolphins" trained by a few navies.

The new weapon combines more effective detection systems, with an "Anti-Swimmer Grenade (ASG). This is basically a concussion grenade with three times as much explosives (1.5 pounds) and a depth setting mechanism. In other words, the ASG can be dialed to detonate at depths from 10 to 100 feet. Combine that with the 20 foot concussion radius and you have a deadly anti-swimmer weapon.

Meanwhile, British, U.S. and Israeli firms have developed special sonar systems for detecting divers. These devices are about the size of a barrel, and will detect swimmers within at least an 800 meter radius. The diver detectors can be linked to automatic ASG dispensers, that will electronically set the depth, then release the ASGs. Divers can't swim deeper than 100 feet, because most ships are not that deep in the water. Even the largest supertanker has a draft of less than 60 feet.

For all the talk about scuba equipped terrorists, there have been very few cases of this sort of thing actually happening. Most terrorist scuba activity have been off the Israeli coast, where the Israelis have been able to deal with the problem without using an acoustic swimmer denial system that makes the diver sick (using nausea inducing vibrations). Either that, or just stay close to them until the surface, which all divers have to do eventually.