Aren't they cute? Except when they're trying to blow you up...

A Soviet special forces diver is parachuted from extreme altitude into sensitive waters. His secret mission is to use high-tech sonar equipment to locate a piece of valuable military hardware that has accidentally splashed down in the wrong place. In the event of meeting an enemy diver, this Hero of the People is equipped with a weapon that will inject his adversary with 3,000psi of carbon dioxide and literally blow him up. Sounds like a scene from a re-make of Thunderball? Well all this really happened, and it gets better. The highly trained operative was a dolphin.

The controversial use of dolphins and other sea mammals by the US Navy has been known about for a number of years, although the precise extent and nature of their activities is still shrouded in military secrecy. But details of the parallel Soviet developments in the field are only now starting to emerge, and they tell a literally fantastic Cold War story. They also beg the question as to whether the Americans have been doing similar things.

The idea of training airborne dolphins, for example, seems incredible. But conservation campaigners have heard the tale first-hand from the former Soviet naval personnel who trained the animals to "jump" from heights of up to three kilometres to avoid detection. Other dolphin "soldiers" were pitched directly from helicopters 50 ft above the sea.

"If I hadn't seen the evidence myself I just wouldn't have believed it," says Doug Cartlidge, a dolphin consultant and front-line campaigner with the Whale and Dolphin Conservation Society (WDCS). He has visited the highly secret naval base at Sevastopol on the Black Sea, home to the once- proud Dolphin Division, to advise trainers on alternative uses for their expertise now that both are surplus to military requirement. While being shown around the unit's museum he saw a full-size model of a dolphin wearing a parachute harness.

"I was amazed at how open they were about the whole thing. But they are desperate for help," says Doug, who once ran the dolphinarium at Windsor Safari Park but has since campaigned for the release of captive dolphins. He was even taken on exercise with the few remaining military-trained animals.

The unit is now part of the independent Ukrainian navy, but there are no funds to run it and a special ship used to transport the animals was recently commandeered by the Russian Navy. The unit has sold off most of its animals to make ends meet. It has also gone into business with a private company to capture more than 30 Black Sea dolphins from the wild for sale to dolphinariums in several countries.

The most controversial of its past activities was the training of "killer dolphins" for use against enemy divers. The US has always strenuously denied that its animals have been used in this way, and even animal rights campaigners have been sceptical about the possibility of doing it. A dolphin is so sensitive to distress signals from divers, they say, that even if it were possible to get an animal to unwittingly kill once, it would not do the same thing again. The secrets of Sevastopol, however, show how the Soviets devised a way of doing just this.

A known use for dolphins by both superpowers was in guarding naval installations. If an underwater intruder was located then they would "report back" to their handlers, acting as an effective early warning system. The Soviet "guards", however, would carry a titanium clamp on the nose, which could be attached to any diver they found by simply nudging them. On the clamp was a device, the size of a ping-pong ball, capable of injecting a high- pressure charge of CO2 into the diver's body. This was activated remotely if a subsequent search failed to locate the enemy.

It has also been reported that Soviet dolphins were trained to carry out "kamikaze" missions. Explosives were supposedly strapped to their backs and they were sent out to blow up enemy submarines. One estimate said that a total of about 2,000 animals had died on such operations. Doug was told that a total of 300 animals had been "tested to destruction" in the Black Sea alone.

The Dolphin Division was established in 1966, following the mysterious sinking of the Black Sea fleet's flagship in Sevastopol harbour. Apart from guard work, other uses include search and recovery following the test firing of navy missiles and torpedoes. Often these could not be detected using conventional sonar equipment - a dolphin's superior system can penetrate up to a metre under the sea bed.

It seems that Doug is not the only one who finds this fascinating. Last month the WDCS detected an attempt to hack into its computers by the Pentagon. The US Navy had previously asked for an advance copy of a report into the trade in Black Sea bottlenose dolphins, which used some of Doug's research. The Americans' own Marine Mammal Program, also once a top-secret affair, has become more visible since the end of the Cold War. Animal rights campaigners there, however, are convinced that the full story has still to be told. One former civilian trainer from the unit claims that killer dolphins have been used by the US, in a wonderfully named "swimmer nullification program".

In 1994 the navy announced that it was to pension off up to 30 of its dolphins, for sale to dolphinariums and leisure parks. There has since been a growing clamour for the animals to be released into the wild and for the whole operation to be closed down. The issue has been the subject of numerous legal actions and in the process quite a lot of information has come to light.

The US programme, known as NRAD, is based at San Diego, California, and was established in 1959 with a single dolphin for the purpose of conducting scientific research into sonar. By 1994 the unit had grown to 123 animals, including 20 sea lions and several beluga and false killer whales used for recovering test-fire hardware from depths of up to 500 ft. At its height, the programme was said to cost $8m a year.

The first recorded use of dolphins on guard duty was in Vietnam in 1970. The only other "operational deployment" that has been admitted involved five dolphins used to protect navy ships in the Persian Gulf for eight months in 1987-1988 during the Iran-Iraq war. The navy has denied that dolphins were used during the Gulf war, but it says the animals have been used for mine-hunting.

The most recent allegation of US military use came in February this year, after the mysterious deaths of 22 dolphins whose bodies were washed up on the French coast. All had a neat, fist-sized hole on the underside of their necks. One theory was that the animals had been part of the American naval operation sending warships to the Gulf at the time of threatened military action against Iraq, and had been killed after "deserting", so their mission would not be discovered.

Who knows? Given the bizarre history of this form of underwater warfare, anything seems possible.