Courses: Concrete socks OK

Floating around at the bottom of a pool, slowly going nowhere, isn't the reason most people take up diving, but it's the essence of the sport. Staying still at depth, rather than flapping around, is the hardest thing to do, even in the current-free confines of the deep end. As with most skills, the only way to improve is through practice, a concept alien to many "qualified" divers.

The Diamond Reef - a few lengths of plastic tubing and some lead weights - can make all the difference. Not only will it make you a safer diver, whether you're a beginner or have years of experience under your weight belt; it will help save the environment, too, by reducing the amount of fragile marine life you crash into.

The system is an underwater obstacle course consisting of one or more diamonds (more like squares) through which you swim. They're made of rigid plastic tubing and are weighted to hang at whatever depth you choose. By the pool side, the squares look absurdly large; below the surface, the course looks a piece of cake and you probably wonder why you bothered to turn up. Then, as you swim slowly through, snagging your tank or depth gauge and perhaps clipping the tubing with your fins on the way out, it dawns on you that not only are you bigger than you realised, but you also move up and down a lot more.

Half the problem relates to diving paraphernalia. You can't possibly fail to notice how ungainly a fully equipped diver is on dry land; those fins were definitely not made for walking, and all the other clobber weighs a ton. The situation magically changes as you submerge, to find yourself weightless and fantastically mobile, operating effortlessly in a beautiful alien environment.

It's certainly the only place I'll ever manage to do a "triple salko with toe loop" without developing a limp, but it can also create a false sense of security. In absolute terms your performance remains towards the lower end of the marine scale, more sea slug than cavorting dolphin.

Doing things such as passing through narrow apertures with a tank on your back is something you can have no feel for until you try it. Though you've entered a world in which you can move in every dimension, you've in fact lost some of your spatial senses. You can see forward perfectly, but can barely look round, let alone behind you to check for clearance. It's like driving a transit van without wing mirrors. But the stakes when diving are a lot higher than scraped paintwork, which is why it's all the better to go through a few plastic squares first, developing catfish's whiskers before throwing in variables such as current and swell.

The other half of the story is more technical, though it's hardly rocket science; we're talking about buoyancy. In broad terms, too much of it and you bob around on the surface; too little, and you get the concrete socks effect. Somewhere in between is your goal.

In practice, when diving with a buoyancy compensator (BC) - a kind of inflatable life-jacket - you're kept afloat to start with by having sufficient air in the BC. Letting some out allows you to sink. Putting just the right amount back in (from your pressurised tank), keeps you suspended, below the surface of the water, neutrally buoyant. Bingo.

The tricky bit is maintaining neutral buoyancy at different depths. Because air is compressible, a given volume in your BC, and lungs will diminish as you descend. As the pressure increases you become correspondingly less buoyant, sinking ever faster. In reverse, just watch your bubbles expand as they rise above you. The effect on an ascending diver, all other things being equal, is exponential: the higher you go, the more the air in your BC expands, which makes you rise faster, which expands the air - and so on. Unfortunate divers who've really lost it come firing out of the water like a Polaris missile, hardly a textbook ascent, which may well be accompanied by the bends or other unmentionable afflictions. It's precisely why good buoyancy control is so critical.

Meanwhile, back at the bottom of the pool, you're going cross-eyed trying to squirm upside-down through a complicated series of squares. The bends is the last thing on your mind and so, ironically, is buoyancy control - it's becoming instinctive, which is just the way it needs to be.

Going through hoops

Contact Matt Crowther (Padi Master Scuba Diver Trainer & Medic First Aid Instructor) of New Concept Diver Training (07970 306 369, or 0181- 644 5357 after 8pm) for details on the BTSI Diamond Reef System, or encourage your diving instructor to take it up.

For holiday divers, an hour in a local pool on the "reef" is an ideal refresher course, before heading off.

A Diamond Reef stamp in your log book is becoming recognised at diving sites all over the world as a sign that you know what you're doing and care about the environment. At his Fiji Island Resort, Jean-Michel Cousteau promotes the system, finding it useful for determining the skills of those wishing to dive on the reefs. Though it's a simple system, the Reef's effect could be fundamental, particularly to the future of heavily used sites where so much of the damage is caused by lousy divers.