Chris - And now to the science of free diving, this is diving as deep as you can but without any air supply. Here to tell us a little more about it is Mark Harris, from the British Freediving Association. Hello Mark.
Mark - Hi Chris.
Chris - So, just in a nutshell, what is the point of freediving?
Mark - It's really to dive down as deeply as you can on one breath of air. There are several disciplines in freediving but the one that most people are familiar with is diving deep by holding your breath.
Chris - You don't have any sort of weights or anything to help you get as deep as you need to go?
Mark - You do, actually, yes. Because you need to wear a wetsuit you need to off set that with a little bit of weight, but it tends to be a little bit less than scuba divers wear, so it's typically around two or three kilos or so.
Chris - So would you swim down, or would you just rely on the weight of the water and the weight you're carrying to drag you down.
Mark - You can do either. Because there are different disciplines in freediving you can either fin down with a pair of fins or you can go on to what's called a sled, which has weights on it, very heavy weights, and that would pull you down. If you do that variety of freediving, which is called no limits, that will actually take you a lot deeper because you don't need to use any, or as much, oxygen to get down there.
Chris - How deep do people who do this tend to go?
Mark - Well up until recently I think the deepest depth was about 150-160...
Chris - Is that metres?
Mark - Metres, yes that's right. I think a lot of people are familiar with Tanya Streeter, a well-known freediver who was hitting those sorts of depths. More recently, there was an Austrian guy, Herbert Nische, and he's just gone down to 211m on a weighted sled.
Chris - Crikey, this is hundreds of feet, how are people doing that?
Mark - It's a combination of things really; partly, freedivers physiology has adapted, people have found new techniques and that sort of thing. And also, the technology that people are using now, for instance Herbert Nische uses a special little bottle that he breathes into part of the way down to help with his equalisation. He doesn't actually use any scuba tanks or anything like that, this is air from his lungs, but he re-uses it in a plastic bottle.
Chris - So how do you prepare yourself if you're going to do one of these dives? How do you get ready to do it?
Mark - What you really need to do is two or three successive breath holds to condition your body. So what a lot of us do is we tend to have a rope in the ocean or lake, depending on where we're doing it, and we just pull ourselves down through about 10, 15 or 20 metres or so. We do a breath hold there for a couple of minutes and then pull ourselves up again. If you do that over a successive period of time it helps to condition your body ready for doing the much deeper dive later.
Chris - And then what do you do, because whenever I dive, to do a few lengths in the pool or something, I find an irresistible urge to want to breathe. Presumably, something is building up in your body. Do you hyperventilate before you do it to get rid of waste gasses like carbon dioxide?
Mark - Absolutely not, no. Certainly not trained freedivers. I think people who don't know much about it have tried that and certain people who go spearing fish do that, but it's just about the worst thing you can do. When you hyperventilate you're blowing off carbon dioxide, and when you do this it causes vasoconstriction of the veins and arteries in the brain. So it's effectively shutting off blood supply to the brain. It also means that when you've blown off that carbon dioxide you've tricked your body into thinking that you don't need to breathe. So you get into an area where you've used up the oxygen in your blood stream, and you get this double whammy effect, which results in something called shallow water blackout. So it's a very dangerous thing really, and it's one of the main things that we try to suggest for people to avoid.
Chris - So a slap on the wrists to me for even suggesting that you might do that.
Mark - Well, not really. I think it's good that you did actually, to alert people to that sort of thing.
Chris - So when you're actually diving, Mark, what's actually happening to your body as you descend to hundreds of feet as these professionals are?
Mark - Well I guess the main thing that you're aware of is the fact that all your air spaces are rapidly decreasing. Because the pressure is building up, the volume of air in your sinuses, your lungs and your inner ear is all decreasing. You feel that as you're going down. Once you hit round about 20m or so, you can stop finning, because you just start sinking, at the same time you have to equalise your air spaces. And then once you get down to past about 40 m you then start having issues with your lungs, because your lungs are squashed right down, they're really compressed to quite a small area.
Chris - Sounds scary. So how do you know when you've reached your limit and it's time to start ascending. That must be critical for survival, isn't it?
Mark - It is, but for everyone it happens to be a different thing. For some people, like myself in fact, it tends to be strength of the legs. I know at a certain depth that I'm going to have problems finning against the negative buoyancy to get me back to the surface. For other people it will be just not being able to compensate the ears against the pressure, that can be one thing. And for other people it's just a mental thing, they just start not panicking but getting a little bit uncomfortable, not feeling happy with the situation. It can be any one of those things really.
Chris - And just to finish off Mark, how long can you hold your breath for these days?
Mark - Well I was in the swimming pool last night, doing some training, and I did a 6 minute 18 second hold, but I have done better than that.
Chris - Good grief, so do you practice in the bath? Do you lay in the bath, underwater kind of holding your nose?
Mark - This is another one of those slapped wrist moments. With freediving, whatever you do, whether it's holding your breath or actually swimming underwater, you never ever do it alone. So doing it in the bath first of all would be very uncomfortable, but second of all I would have to have someone with me to monitor me.
Chris - But as they say, baths are always fun if there's more than one person in there at once.
Mark - Well, that's true, yes, there is that to think about, and there's the rubber duck to play with. You could do it in the bath; it's a little bit tricky. For me, personally, if I'm training at home I tend to do it out of the water and just do dry breath holds.