Trainers at a long-established Naval landmark in Gosport are used to working under pressure – quite literally.

Since 1954, in their safe hands, every potential RN submariner has had to undergo the rigours of the ten-storey, 30-metre (100ft) deep Submarine Escape Training Tank (SETT) before they can qualify.

Among those who went through the system was the Second Sea Lord himself, Vice Admiral Sir James Burnell-Nugent, who went on to command submarines HMS Olympus and Conqueror.

The admiral was among those who witnessed a demonstration of escape drills and breath-held techniques at the SETT, located at Fort Blockhouse, to mark its 50th anniversary.

Around 2,500 submariners a year are currently trained in the tank, which was commissioned specifically to teach submariners escape techniques in the unlikely event of a submarine accident.

Instructors - all submariners themselves - undergo a three-month training period, followed by a continuous development programme of up to 18 months.

“We teach them a technique of breath-holding, so they can, if necessary, move through the water quickly without causing bubbles, which would completely obscure the view of other safety staff,” explained Officer in Charge SETT Lt Cdr Bob Mannion.

Students start off making ascents from nine metres below the surface, graduating to 18 metres where, once the pressures were equal, the lock doors were opened at the demonstration, and the submariner emerged, exhaling all the way in the warm water to the top of the tank, which can hold 150,000 gallons.

In a simple but chilling display of the effects of the change of pressure, the bag from the inside of a wine box was floated up, compressed to a third of its original volume, gradually emerging unscathed at the surface. Another, incorrectly compressed, rose rapidly and broke the water, flattened, in an uncontrollable fashion.

“That’s what a person’s lung tissue would look like - no nerves around the lungs,” said Lt Cdr Mannion.

Ultimately, students are taken to the foot of the tower, where they ascend using an orange immersion suit, replete with its own dedicated life raft – another of the techniques demonstrated on the day.

The Second Sea Lord, who had his first experience of the SETT in 1973 and, like all his fellow submariners, had to re-qualify periodically, recalled it as “a difficult day”.

“You know it is going to be painful on the ears, and, against your instinct, you have to breathe out. It’s a classic case of mind over matter,” he said.

“There is also, of course, peer group pressure - no one wants to be the first person to opt out.”

The equipment has changed over the years, he added, with constant improvements in design and techniques.

And safety? As someone said on the day: “With all our training, it’s more dangerous driving out of here than being in a submarine – especially in Gosport…”