Slowly, I sink beneath the surface of the frozen Tignes Le Lac in the heart of the French Alps and feel a cold, sinister squeezing around my rib cage as the pressure of the icy cold water increases. My breathing becomes more laboured, and the sound as I inhale and exhale through my regulator is amplified inside my neoprene helmet. Expelled air bursts forth in silvery bubbles and races upwards to become trapped under the ice sheet above me.
Unlikely as it may seem, I'm on a trip to one of the biggest ski resorts in the world and have somehow found myself being talked into having a go at ice diving. I wouldn't recommend it to anyone suffering from claustrophobia - it's cold, it's confined and if you don't like it there's no quick escape route.
But it is a remarkably beautiful underwater world all the same.
My guide and instructor David Monmarche hands me a waterproof torch and invites me to watch the air bubbles as they bounce off the icy roof above and scatter in all directions like globules of shimmering mercury. Those that fail to find their way to the hole through which we entered the frozen lake eventually coagulate and freeze to form pretty stacks like translucent dinner plates.
There's a strange, cold light down here, a diffuse icy blue that scatters through the opaque surface of the ice and then bursts through our entrance hole like a spotlight. As we weave around beneath the frozen surface, my hand brushes its underside and glinting daggers, glittering needles and shiny clear spicules break free and drift past.
I've only dived once before, in the warm, turquoise waters of the Red Sea, and this is about as similar as walking on the moon. Plunging through a small, dark hole in a frozen lake high in the wintry French Alps may seem like the act of a masochist, but it actually isn't such a bad option today. Most of the ski slopes are closed due to avalanche risk from the two-day blizzard we're enduring - since the skiing options are limited why not slide beneath a metre-thick layer of ice for a submarine chill out?
Anyone can do it, or so the adverts say. You don't need to have dived before, and Monmarche is a qualified Padi instructor who runs me through the basics that I need for my 20-minute underwater excursion in a matter of minutes.
Water temperatures in Tignes Le Lac are generally around 4 degrees Celsius once under the frozen layer of surface water (unlike seawater, which can remain liquid when it's as cold as minus one degree). Although the water is a little above freezing, it's still cold enough to make a drysuit necessary rather than a wetsuit. As the name suggests, a drysuit prevents any water making contact with your body, and thus keeps you warmer than a wetsuit.
Having clambered into my drysuit in a small, overheated hut beside the lake, as I follow Monmarche out onto the ice snow falls heavily around us. I'm kitted out with fins, weight belt, mask and regulator, then wait with some trepidation as he breaks the pancake thick layers of ice that have begun to form over our entrance hole before sliding into the steel-grey water like a seal.
I'm then connected to him by a length of climbing rope, which in turn is connected to the surface, and as I join Monmarche in the water I'm surprised that it doesn't feel that cold. Well, not at first.
You're aware that your body is immersed in a frigid environment, but the drysuit does its job pretty effectively apart from the occasional drip of cold water that forces its way in and feels like the Ice Queen's index finger running down your spine.
Once I'm orientated and feeling relatively comfortable, Monmarche invites me to explore on my own. I look down to the bottom of the shallow lake (ice dives for beginners rarely descend more than a metre or two below the surface, and are short, so all the complicated stuff like decompression is unnecessary). The bottom is visible and consists of glacier-smoothed boulders adorned by weeds, and momentarily I think of paying it a visit. But I'm content to float around gazing at this remarkable cold, crystalline environment, absorbing the array of cool greens and icy blues, and the shafts of silvery-white light that occasionally flash from a drifting splinter of ice.
My dive is a far remove from the experience of professional ice divers, such as those working with the British Antarctic Survey. At its Rothera base (the only base in Antarctica that supports diving through the entire Antarctic winter, when surface temperatures regularly fall below minus 30 degrees) most of the dives are conducted through holes cut with a chainsaw in ice that may be as much as two metres thick. Two dive holes are always cut so that if a seal decides to use one of them, the divers have an alternative exit. The base has a state-of-the-art recompression chamber, which is always primed and ready to treat divers with the bends, which can occur when they ascend too quickly.
Experienced ice divers head to locations such as Russia or the Norwegian fjords for the excellent visibility and unusual sights such as giant king crabs and second world war wrecks. The divers remain connected to a partner on the surface with a rope, and must have the skills to dive without getting entangled in it. Breathing regulators are designed so as not to freeze, and rope signals (one pull for "yes", continuous pulls for "emergency") are used to stay in contact with the surface. Among the various special techniques that are required is how to hit the underside of the ice if your dive belt falls off and you rise to the surface uncontrollably.
Fortunately, I had no need of any of this, and felt happy to be a complete novice, guided around a bizarre mountain world I previously never knew existed.