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They call themselves "sub hunters", men and women trained to detect enemy submarines gliding in the depths of the world's oceans.
Like Cold War spies, they use a mix of state-of-the-art technology, rigorous training and split-second intuition to find out where the hidden subs are, where they may
be headed and how fast they are going.
The Cold War may be long over, but the demand for the trade is as strong as ever.
With 540 submarines in operation around the world, many in the hands of what the British military refers to as "potential enemies", training officers say the threat remains as real today as it was three decades ago.
That has been underlined in recent weeks with Russian President Vladimir Putin reviving the Soviet-era practice of sending bombers out on long-range patrols, a move interpreted in the West as sabre-rattling by its erstwhile enemy.
"The threat now is far more diverse than it was during the Cold War," said Master Aircrew Steve Street, a member of the team that instructs the top sub hunters and which last week cautiously opened its door to the media for the first time.
The timing of this - just days after Putin's announcement - may be coincidental and the nature of the threat different, but the cat-and-mouse game between submarine and sub-hunter was as much a Cold War feature as that between bombers and fighters.
"Even if a British ship has not been sunk since the Second World War ... the submarine threat is still incredibly significant."
To counter this, Britain's Navy and Air Force run what's called the "A" course, an elite training camp at a high-security compound near London for experienced sonar operators from Britain, Australia, Canada and other allied countries.
The month-long course, dubbed the "Top Gun" of the sonar world, teaches up to two dozen trainees the most advanced techniques for detecting submarines amid the cacophony of marine life and merchant shipping emanating from the ocean depths.
Watching graphs of sonar patterns and listening to sounds captured by underwater microphones, the trainees learn to separate merchant ships from trawlers, the noise of whales from that of seals, the creak of pack-ice from the screech of icebergs - and amid it all the hum of submarines.
What's more, a trained ear can quickly calculate not only what type of sub is moving through the water, but how fast and in which direction. The best can identify individual potentially hostile subs merely from their acoustic pattern.
"We call it the 'black arts'," said Richard Horsburgh, an aural analysis expert and "A" course trainer who spent 25 years as a sonar specialist in the Navy, including on submarines.
"It's a game of cat and mouse. You build up a database in your head so you can identify almost anything.
"When I was in the subs during the Cold War, we'd patrol the oceans for weeks at a time, sucking up as much acoustic information as we could get, and then sneak off."
Training officers declined to identify the source of a perceived post-Cold War threat, but more submarines have been sold on now by major powers such as Russia and China to countries including Venezuela and Iran.
Russia has about 70 submarines and while patrols have plunged since the mid-1980s, defence journals say they have picked up again in recent years.
These days sub hunters still go down in Britain's 15 nuclear submarines, but also work in co-ordination with the Air Force and its fleet of Nimrod patrol aircraft. Surface ships and some helicopters also have sonar operators aboard.
The Navy sees the "A" course as the best way of building up skills to steadily improve the chances of correctly identifying the threat that may be out there.
About 20 per cent of Britain's sonar operators have done the course.
But long before they start identifying individual submarines, the first thing sonar operators learn to detect is the sound of fishing trawlers - no submarine captain wants the ignominy of being caught in the net of a fishing boat.
"If you ever hear trawl noise ... forget about subs, " said Mr Horsburgh. "You have to get away."
- Reuters
