The next terrorist threat may come from the deep. In recent years, several homeland security alerts have focused on the danger of scuba-equipped terrorists targeting docked Navy vessels or ocean-side nuclear plants. Now the U.S. military is quietly developing a new generation of underwater weaponry capable of warding off undersea trespassers with liquid bullets.

It's a revival of an underwater arms race that was hot during the Cold War, when Western scientists sometimes struggled to match Russian technology. The Soviet threat was estimated to include some three thousand Special Forces frogmen – an opponent virtually impervious to traditional arms.

Normal guns will work underwater, but the drag slows bullets right down. "I have tried it myself in our pool," says Scott Greenbaum, a Certified Glock Armourer and webmaster at GlockFAQ.com. "The bullets only traveled about 15 feet."

In addition to the drag, firing underwater is hazardous: Some types of ammunition can burst the gun, and the shockwave from the muzzle blast can cause permanent hearing damage. The Glock 17 is one of the few weapons that can be customized to fire underwater, with the aid of maritime spring cups, which stop water from impeding the firing pin.

But the amphibious glock is designed to be carried, not used, under water. "I've never heard of anyone actually shooting a shark, or fish, or person, or anything," says Greenbaum. So in 1970 the U.S. Navy introduced a special weapon for the job: a chunky six-shooter called the Underwater Defense Gun, or UDG.

Instead of firing bullets, the UDG "fired a stiletto-type dart that could provide range, accuracy and lethality underwater," says Tom Hawkins of the nonprofit Naval Special Warfare Foundation. To reduce shockwaves, a pusher piston sealed the barrel after firing. Each barrel could only fire once, hence the need for six separate barrels and the weapon's chubby profile.

The effective range was about 30 feet. As a bonus, the subsonic projectile and sealed firing system made the gun virtually silent above water.

"The weapon worked quite well," says Hawkins, "but it was bulky and heavy and the men never warmed up to it. It was also a signature controlled item – as in signing your name to check it out – and administratively a burden to the operational units."

At around the same time, another approach surfaced in the privately developed Gyrojet pistol, a James Bond weapon that fired miniature rockets (Bond even uses one in You Only Live Twice). Unlike bullets, rockets accelerate after firing and experience less drag. But the Navy found the underwater rockets inaccurate, only hitting a human-sized target half the time at 30 feet. It was not adopted.

Steve Ritter, a weapons designer and Gyrojet expert, believes the testing was unfair. "Accuracy was an issue, but it appears that most of this problem was an ammunition quality-control problem."

Meanwhile, on the other side of the Iron Curtain, Russian engineer Ivan Kasyanov of the Klimov Institute had found a revolutionary solution to the problem: a projectile which he called a “flying nail.” It had twice the range of the U.S. equivalent, its secret being a blunt tip instead of a stiletto-point.

Underwater, a blunt projectile forces the water away on either side of it. At a high enough velocity, this causes such low pressures that a cavity forms. This cavitation decreases the drag on the rest of the projectile. Supercavitation, in which the entire projectile apart from the tip is enclosed by the cavity, hit the headlines in the '90s with the advent of the Russian 200-mph Shkval rocket torpedo. But the smaller projectile had already been around for years.

The SPP-1 flying-nail pistol came into service in 1971. It is more compact than the Underwater Defense Gun and half the weight, and even now is standard issue for Russian combat divers, including a section of the Presidential Security Service that patrols the flooded sewers connecting the Kremlin to the Moskva River.

An even more formidable flying-nail weapon followed in 1975, called the APS or Special Underwater Assault rifle. "The APS is a lot like a Kalashnikov actually," says Russian defense analyst Viktor Litovkin. "It has a rate of 500 rounds per minute."

The APS has an underwater range of a hundred feet, the practical limit of visibility in good conditions.

The latest Western technology in use is the Heckler & Koch P11, which appeared in 1976, and is wielded by U.S., British and German special operations forces. Contrary to rumor, it does officially exist, but the technical details are kept secret, even more than 30 years after the gun was first issued.

"The pistol P11 is a classified weapon according to regulations of the NATO and the German government," Marc Roth of Heckler & Koch told Wired News. "Due to this we are not authorized to reveal or confirm any technical data of this weapon."

What is known is that the P11 fires a heavy projectile at fairly low velocity, with an effective range similar to the Russian SPP-1. It seems likely that the bullets are cavitating, as Germany has long been a leader in this area.

Today, the first line of defense against underwater terrorists – should they emerge – will be sonar and depth charges, rather than armed frogmen standing guard. But anyone tangling with combat divers will find them very well armed, and U.S. scientists are developing the next generation of underwater weapons. Much of their work relates to larger projectiles for mine or torpedo defense, but is also applicable to small arms.

Chris Weiland of the Advanced Experimental Thermofluid Engineering Research Laboratory in the mechanical engineering department at Virginia Polytechnic Institute is looking at new ways of achieving cavitation by injecting pressurized gas into the path of the projectile. "Natural cavitation only takes place at very high speeds," says Weiland. "This can be dramatically reduced."

Once a supercavitation bubble is established it can be maintained by venting rocket exhaust into it ("ventilated cavitation"), a system employed by the Russian Shkval torpedo. Developers of a modern Gyrojet pistol could use this method to give it greater range and power than any existing underwater weapon.

Perhaps the most intriguing hint of where the underwater arms race is headed comes from a 2005 U.S. patent granted to Thomas J. Gieseke, a Navy scientist at the Naval Undersea Warfare Center. The patent proposes a "high-velocity underwater jet weapon" that fires a stream of high-velocity liquid "bullets" – fine grains of metal or sand that form a cavity more efficiently than solid rounds.

Gieseke declined to comment on the research, which gives a whole new meaning to the term "water pistol."