"Underwater warfare is back in the news. This time, however, it's not just hostile states that governments are worried about. In Sri Lanka, the Tamil Tigers have already shown the potential for underwater sabotage in raids on government-held harbours. Security services in the Netherlands claimed in 2002 that terrorists were learning to scuba dive in Tunisia, and the FBI has announced that it believes Al-Qaida is training divers.

The concern about terrorism has prompted a rethink. Netting off all the undersea pipelines, nuclear power stations, refineries and even tourist resorts that might be a target is impossible. There are for example, at least 40, 000 kilometres of pipeline in US waters alone. So what is the answer?

By far the most advanced solution uses sound waves like those emitted by sonar. The latest commercial sonar systems emit pulses at frequencies between 10 and 300 kilohertz and are precise enough to spot a diver up to kilometre away, no matter how murky the water. If these pulses are emitted at high enough intensities they can also make an effective weapon. In 1993 a US diver was left temporarily disorientated and numb during sonar tests in the northern Mediterranean, and more recent experiments have shown that divers exposed to sonar can also experience nausea and dizziness."