The vehicle is still a fiction, but the technology that could produce it is real enough. At least one Russian submarine is armed with torpedoes that exploit it. The same idea is being used to make guns for destroying underwater mines, and it could double the speed of surface vehicles such as hydrofoils. Most remarkably, it could produce an underwater vessel capable of traveling at thousands of kilometres an hour while remaining almost entirely dry. For the key to this technology lies in the humble bubble.

The problem holding back ordinary torpedoes was drag. Any object, no matter how streamlined, suffers resistance as it moves through a fluid. In the early 1960s, Mikhail Merkulov at the Hydrodynamics Institute in Kiev realised that the solution lay in a phenomenon called cavitation. It was a daring idea, because naval architects usually see cavitation as a menace, rather than something that works to their advantage. But supercavitation is a different matter - under certain conditions, a single bubble or supercavity can be formed, enveloping the moving object almost completely.

Supercavitation is hard to achieve - the body has to be going pretty fast, at least 180 kilometres an hour, or 50 metres per second and the shape of the nose also has to be right, say researchers. Rather than being streamlined, a supercavitating object needs a flat nose. Then, at high speeds, the fluid is forced to flow off the edge of the nose with such speed and at such an angle that it can't wrap around the surface of the body. A supercavitating body has extremely low drag, because its skin friction almost disappears. Instead of being encased in water, it is surrounded by the water vapour in the supercavity, which has much lower viscosity and density. So in a supercavitating vehicle, only the nose of the craft causes significant drag, because this is the only part of the body actually in contact with the water.

According to Mark Galeotti, a defence expert at Keele University in Staffordshire, prototypes of the torpedo appeared in the 1980s, but there was still plenty of work to be done. "It was only in the early 1990s that the Russians were in a position to produce a proper working torpedo," says Galeotti. Called Shkval, meaning squall, it is said to be capable of speeds as high as 500 kilometres an hour. It is fired like an arrow from the submarine, possibly with a mechanical catapult. That gets it moving fast enough for the cavity to form, so that its rocket can be lit. The Russians had built themselves a formidable weapon. Shkval leaves enemy torpedoes standing, and can knock out an opposing sub before it has time to react. It could even be used defensively to intercept enemy torpedoes.

Just a few years after Shkval's debut, NUWC (USA) researchers announced they had gone supersonic. An unpowered projectile, with a carefully designed flat nose and fired from an underwater gun, broke the sound barrier in water. That's nearly 5400 kilometres per hour--or 1.5 kilometres per second. Lacking any onboard power to sustain its motion, the shell slowed rapidly, but this was still a vivid demonstration of the speeds that supercavitation makes possible. Already they aren't very far off the 2.5 kilometre-per-second speed record for conventional munitions in air, and scientists have calculated that their supercavitating projectiles should be able to match or even surpass this.

But there are plenty of technological hurdles. One thing still needed is a powerful and compact propulsion system. An aluminium-burning rocket is one answer. It would use water as its oxidiser, and so wouldn't need to carry oxygen. The problem with aluminium has been that unreacted fuel quickly becomes coated with aluminium oxide, inhibiting any further reaction. To avoid this, powdered aluminium can be injected into a vortex of water, which keeps the burning, molten drops apart. Aluminium-burning rockets may be fine for short ranges, but what about long-distance supersonic underwater travel? For this, probably only a nuclear reactor is a compact enough power source. Traveling at 2.5 kilometres per second, such a craft could make the journey from London to New York in well under an hour, easily outpacing Concorde. Assuming it didn't hit a whale on the way, of course.