I have two oxygen tanks strapped to my back and I'm halfway up a mountain in Switzerland. My dive buddy and I waddle in our flippers to the lake's edge. The temperature is several degrees below freezing but the pistes are packed; the snow is deep and the skiing is good. We check our mouthpieces and brush the snow off our facemasks.
My buddy taps me on the shoulder. It's the signal to sit on the pool's edge. Above us, skiers look down from an Alpine gondola. Another man gives us a thumbs-up - the signal to make our dive. The dark depths of the glacial lake in the shadow of the Matterhorn await us.
Ice diving is the latest winter sport, endorphin-surge fad and adrenaline fix. The Zermatt Ice Rock Club is the world's first, and as yet only, ice-diving school and is run by French former marketing executive Gerard Garson, who gives me my dive signal.
For 400 Swiss francs (HK$2,500), all equipment and limitless oxygen included, you can learn to dive under the snow 3,440 metres above sea level in mid-winter in the middle of Europe. A two-day course consists of three 20-minute dives in a frozen-over glacial lake. The school is open from December until the end of April.
'It is the only sport where two pounds of high explosive are required before you can even start,' says Garson. 'Every time we dive we must first get permission from the local authorities, then clear a hole in the ice using a small charge of dynamite. Safety is paramount before, during and after the dive.
'Preparing for the dive is the most exhausting thing. Shovelling and brushing away all the exploded snow cover so we have a safe snow hole isn't easy, especially at altitude,' he says. 'We get some funny looks from passing skiers. Sometimes we go into bars in town wearing our scuba outfits.'
Fabrice Bourgand, a 32-year-old former French Navy diver, is my instructor and dive buddy. He recently set a world ice-diving endurance record by spending two minutes and 33 seconds under the ice, swimming 60 metres at a depth of seven metres without oxygen. The underwater temperature was minus 10 degrees Celsius.
'The rush from ice diving is incredible. It is the ultimate adventure sport. You have to be extremely fit.
You get to the dive site by cable car and Alpine gondola, and then a snowmobile,' says Bourgand. 'If the snowcat is out of action you have to walk. You have to be committed. Very committed.'
Taking a deep breath to brace ourselves against the death-chill, we slip through the three-metre-diameter hole in the ice of the Lac du Glacier Theodul. The intense cold grips me immediately. Feeling crushed by it, I gasp loudly and bite the mouthpiece to control the shock. Then I begin to tread water.
After some initial disorientation I find myself in the middle of a cloudburst of milky foam, which swirls slowly around me. Tiny bubbles, like spangled gems, rush upwards in front of my face. My first thought is that I am in the middle of a snowstorm.
Suddenly, everything is calm and my eyes adjust to the curious blue-white nothingness around me. The high-altitude mountain lake seems to be ours alone. Bourgand and I are tied to a lifeline, which snakes out behind us up to the surface and through the hole.
But it's not all silence. Occasionally there is a creak and a groan as the ice moves around us, shifting as if making room. The light refracts through it, creating a strange, kaleidoscopic laser show through the water. The hole above looks like a mirror on a wall. As we kick on - slowly to avoid tiring ourselves - I notice that we are in a frozen vault, between four enormously tall and seemingly unending walls. I cannot see the bottom.
It is like swimming inside a giant ice cube. I have never felt so hemmed in in my life. We swim forwards and touch the sides of the cavern. Ice splinters off in our hands and hangs as if in mid-air in front of us. I feel a tug on the line and turn around. Bourgand is pointing at his watch and signalling that it is time to return skywards.
As we surface, the harsh light and return to full-colour vision make me squint. The sunlight stings
my eyes. Pockets of extreme cold bubble up from beneath my wetsuit and my whole body is covered in goosebumps. I feel myself being lifted onto the ice and put in a snowmobile, which takes me to a small lakeside hut that passes for the Zermatt Ice Rock Club's changing room. Someone hands me a cup of coffee. I am exhausted, but exhilarated.
Bourgand smiles. 'Everyone feels the same. A mixture of euphoria and pneumonia!'
Getting there: Lufthansa (www.lufthansa.com ) flies from Hong Kong to Geneva via Frankfurt. Zermatt can be reached by train from Geneva. See www.eurorailways.com . To organise an ice dive, visit www.evolution2.com .