It's said that in the execution chamber as the gas rises, the condemned man will take a penultimate deep breath, filling his lungs and bloating his cheeks. However long he can hold that breath is the measure of his remaining time here on Earth.

At the start of our lives breath-holding comes naturally. it's perfectly defined by the British Gas TV ad and the seminal Nirvana album cover showing infants seemingly so at home underwater. Witness the market for newborns swim classes. But we quickly lose the knack.

Now, as a fully-fledged adult, I'm going to try and get it back. And I've set myself the goal of proving it by reaching a depth of one hundred feet on a single breath of air. Ill make my attempt in some of the warmest, clearest water to be found anywhere in the world. Gosport, on the South Coast of England.

The Submarine Escape Training Tank (SETT) is probably the deepest indoor pool that's open to the public. It was built in the fifties to allow submariners to practice the emergency ascent skills that could save their lives in the event of a submarine accident. Through airlocks at the tower's base trainees rush to the surface. Between the bottom and 10 metres the air in their lungs will double in volume. Between 10 metres and the surface it will double again. The ride takes just seconds, as the submariners are borne aloft by the buoyancy in their escape suits. To prevent their lungs exploding, the escapees must exhale - hard.

My time at the tank will be very different. I've got just one day to get my body and mind into shape for a single dive. One that will take me on a round trip equal to the height of two ten-storey tower blocks. To do it Ill have to suspend breathing for a full two minutes.

Howard Jones is the man who just might make it happen for me. Jones is the catalyst for the British Freediving Movement. He greets me at the tower with a firm handshake, befitting a man who was once a Royal Marine. Hes compact, very fit, and seemingly laid back. But that is an illusion. Howard Jones is on a mission. He touches on evangelical in his zeal to bring the sport of freediving - diving only with the air you can carry in your own lungs - to the public at large. Hes been extremely successful. Jones set up the British Freediving Association, founded the magazine Freediver and featured prominently in the QED documentary "Call of the Deep" on BBC television, much of which was filmed at SETT.

The tank is housed in a tower block. Gosport is a Navy town with few high rise buildings to serrate the skyline. So SETT stands out, especially if you are confronting it for the first time. Once inside, and following a quick briefing on dos and donts from an officer, we take the lift to the tenth floor.

The tank stands up from the deck like a very ordinary above-ground swimming pool might in your own garden. it's only about 6 metres in diameter. But look over the edge and it is just awesome. The water is so clear it's like looking down through green tinged air. The circular tank seems to converge at the bottom. One hundred feet down I can see the hatches through which the Submarine Service pass on their way through that high speed, pressure dropping joyride to the surface. To stop them bouncing off the walls, the support divers clip them to a central steel hawser that runs all the way to the roof above us. For me, it's downright intimidating. I pull on my mask, snorkel and fins and swing over the side and into the water. The first surprise is when I let go of the side and dont immediately freefall to the bottom. The second is how warm the water is. Trunks are all Ill need for the six hours Ill spend in the tank. I take a couple of breaths, upend, and fin down. I've been a snorkeler for 30 years and easily make the 15 metre level. I'm amazed to coast through a thermocline - a physical layer that divides the very warm water above me from the merely lukewarm water below. The difference is probably only a couple of degrees. Outside it is a grey November day. Indoors it's warmer than diving in the Maldives, and clearer. The deepest I've ever achieved in the sea is 18 metres. In the relaxing environment of the tank it isnt long before I've dived way deeper than that. Soon I've plunged to 25 metres to admire the mermaid painted with love on the tank wall. Jones isnt happy with me though. Hes been sat in the SETT commanders chair like Blofeld, but without the cat. From here he controls two closed circuit television cameras inside the tank. Hes studied my ascent technique and offered hard won advice on how to surface more easily. Easy is a watchword for freedivers. Pausing your body's breathing for a couple of minutes isnt natural. And your metabolism does not stop. It keeps going, burning oxygen and creating carbon dioxide. Effort, however little, increases your metabolism. Freedivers do everything possible to reduce effort and so prolong their time underwater. Jones is becoming unhappy with all the dives I'm making. For a start it is my first time here. Given this wonderful science fiction set, I cant get enough. I've no sooner surfaced from one dive, than I'm off making another. Good freedivers dont make dives in rapid succession. After each dive, they relax at the surface, letting their bodies recover from the abnormal demands they have placed on it over the last few minutes. it's a practice many athletes will recognize. I'm not doing any of this. Eventually after a couple of hints that I should slow down, Jones simply orders me to take a break. Hes a nice guy and I dont argue. I'm not fazed that he used to box for his unit.

By the late afternoon I'm ready to take a crack at making the bottom. It will be my first dive to such a depth. And I'm going to come a bit of a cropper making it.

I begin by trying to relax at the surface. With my face in the water and my eyes closed I spend a few minutes just breathing normally. Then I take three deep breaths. I'm drawing air right into the lower lobes of my lungs, then expelling it. I'm trying to maximize the amount of clean air in my lungs. Usually we only utilize a small part of our lung volume. For this dive I want as much oxygen as possible. I'm also trying to reduce the carbon dioxide in my system. it's the build up of carbon dioxide that urges us to breathe, not the lack of oxygen. Reduce the CO₂ and you delay the stimulus to breathe. On the last breath I gulp in as much air as I possibly can. it's called air-packing. Essentially I'm overfilling my lungs and it isnt very comfortable.

Then I jack knife and head for the bottom. A couple of kicks of my near metre-long fins and I'm speeding downwards. My thumb and forefinger are balled under my nose, elbow locked to my side for streamlining. Every few metres I exhale gently against the seal made against my nostrils. With nowhere else to go the air is forced from my throat into my middle ear. It equals the increasing water pressure building on the other side of my eardrum and prevents it bursting. Then I breathe out slightly into my mask. If the air inside my mask, an air space like my middle ear, isnt in equilibrium with the water outside, gross things will happen to my face. My body is mostly water and automatically balances with the pressure bearing down on it. But my mask is rigid. As pressure mounts in the tissues of my face, theyll swell and bulge into the air space formed by my mask. At best Ill surface with a couple of black eyes. Exhaling will equalize the pressure in my mask with the surrounding water and my own body tissues. So no shiners.

At just 5 metres my lungs are collapsing nicely. I dont sense this, other than I no longer feel the discomfort of my air-packing seconds ago. But as the volume of my lungs reduces, so does my buoyancy. I'm falling fast now under my own dead weight. I cross my fin tips and assume as streamlined an attack angle as I can. I'm picking up speed with each metre I sink now. 10 metres goes by, then 15, then 20. At 25 metres I'm feeling the first pangs of air hunger. At 27 metres my mask slams solidly into my face. I try to exhale and dont seem to have the air left to do it. I've crossed a physiological line, and I know it.

I high five the metal floor, one hundred feet down. My lungs have reduced to one quarter of their surface volume. Arteries in my chest have engorged with blood to fill in the gaps. All I have to do now is ascend.

My dead weight got me down here with virtually no effort. Now I must kick that dead weight to the surface, one minute away. I push off the bottom and kick hard towards the light. From this depth that light is just a tiny circle far above me. The air that I used to equalize my mask is now expanding as the pressure drops. A froth of bubbles escapes from it. I leave them behind. Dancing in the beams of the pool lamps, theyll reach the surface long after I do.

The air that had been compressed inside my lungs is also re-expanding. One effect of this is that the oxygen that was forced into my bloodstream with increasing pressure is now leaving it. And there is less of it because it has been keeping me conscious during my dive. The building carbon dioxide waste gas that first warned me that I needed to return to the air has also flooded back into my lungs. With it goes the urge to breathe. it's a bad sign. it's a long, long way back. The 20 metre band recedes behind me to be replaced by the 15 metre mark. Half way. Then the 10 metre band. Then five. At five I'm becoming buoyant again.

I break surface, exhale and draw a sharp intake of breath. Tremors run through my body. They are fleeting. Just an uncontrolled series of shocks like using a muscle tone exerciser. it's called a samba. And it means I've overdone it. The next stage would have been a blackout, either just before or just after surfacing. it's the freediver's greatest threat. Many have died, often while spearfishing. Competition encourages greater risk taking. Not long ago the author of the breath-hold corps bible, "Freediving", Terry Maas, lost his 19 year old son Loren to a snorkelling accident. Last year Audrey Mestre died trying to set a new depth record. To engage safely in this sport proper training and restraint is essential.

I've set a personal best. The world record is over 500 ft and held by somebody else. That suits me just fine.