They can hold a breath for more than five minutes. If that were not remarkable enough, they can swim to depths most other divers would refuse to go, even with an aqualung.

It is highly dangerous, of course. They risk blackout, brain damage and even death by starving their bodies of oxygen for so long.

No diver is allowed to go deeper than 60m at the world championships

But whereas most of us come up gasping after only a few seconds under water, these men are able to push back the physical barriers and experience something that they even claim to enjoy.

"The fear or sensation that you need to breathe only lasts five or 10 seconds," says Howard Jones, who captained a British team at the free-diving world championships in Sardinia this year. "But most people would come up because they don't know that after that it becomes pleasurable.

"We get this pleasure zone and then you lose time. It's tranquil and you've no idea that you're holding your breath."

Howard and his team were featured in a QED science documentary broadcast on BBC One on Tuesday.

Dive reflex

Before they left for Sardinia, the team were subjected to a series of unique scientific tests at the Diving Diseases Research Centre in Plymouth. Under the eye of Dr Maurice Cross, it was established that the men have what is called the "dive reflex".

Free divers can drop their heart rates and conserve oxygen

It seems that, just like seals which can go 45 minutes under water without breathing, the heartbeat of the divers will drop dramatically the moment they immerse their faces in cold water.

"There are receptors in the mouth, around the snout, which, in this group of men, probably go to an increased active reflex, so that when they get cold water immersion, they immediately behave a bit like a seal - and their heart rate drops off."

Howard's own heartbeat drops to a quarter of its normal rate. Dr Cross says this helps a free diver conserve the oxygen already in his system.

Dr Maurice Cross: Astonished

"The oxygen that you have in your body is very precious and there are only two places you want it to go - your heart and your brain. So they just shut down the rest of the circulation and stop pumping blood around the body where they don't need it."

What Dr Cross found truly remarkable, however, is the way the divers are able to cope with these very low levels of oxygen. In human blood, the oxygen saturation level is usually about 98% and doctors begin to get very worried if it drops below 90%.

Anyone in this condition can usually expect to find themselves in hospital breathing in an oxygen tent. Our "mermen", on the other hand, can watch their oxygen saturation levels drop to half their normal level with no apparent ill effects.

Free-dive pioneer

It is clear we still have much to learn about how the body functions in extreme situations.

There is always the risk of blackout

It was this desire to find our limits that drove one of the pioneers of free diving, Jacques Mayol.

The Frenchman, whose exploits were celebrated in the film The Big Blue, developed new techniques for diving deeper and deeper on a single breath.

He even studied the anatomy of dolphins to help him explain how the human form might survive the crushing pressures that exist more than 100 metres deep.

Mayol went there and beyond. Others who followed went further still.

But make no mistake, free diving is an extremely risky business and today's world diving authorities frown on all attempts to set new endurance marks.

This is not something you try out "to see what it is like" - not even in the bath.