The water's two degrees and my mouth, the only part of me exposed, is now numb. It's of no mind, though; the extreme environment is too beautiful to be worrying about small discomforts. I lie still, occasional wafts of my flippers my only movement, drifting in a current through the thin beginning of Iceland's Silfra Fissure in Thingvellir National Park, as I prepare to glide between the Eurasian and North American tectonic plates. It's a tranquil time, silent and pure. I've managed to get ahead of the rest of the group, so no flippers in the face can ruin my view, which extends all the way to the bottom, 100m in some places. Adjusting my snorkel causes an intake of half a mouthful of water, but swallowing it solves the problem and is probably good for me; the liquid that fills Silfra is melted ice from a glacier 50km away, filtered through the country's porous lava fields for many years before arriving at the north end of Thingvellir lake through underground wells. It's likely the cleanest on earth.
Above me, the sun has failed to pierce the brooding sky and the water's surface remains unmolested by wind; perfect conditions, as shards of light and the slightest gust would affect the stunning clarity, rated among the world's very best for diving and snorkelling. I float on, taking care not to touch the sides, lest I stir up the plant life, ruining the view for those behind me. Below me lies the cathedral, an area where the Icelandic diving club holds its annual Christmas ball. Regrets at not having got my PADI certificate surface, as my snorkelling dry suit is too buoyant to delve any further than the surface.
Before reaching the lake proper, I take a left turn and enter the lagoon, an expansive area in which our guide let us roam as we wish. Snorkelling from one side to the other, then paddling along the banks, I marvel at the rock formations and strange, placenta-like growths and in places reach out to touch the bottom, only to be reminded that it's too far down, the immaculate visibility fooling me yet again.
It's been 15 minutes since my last conversation with anyone and I pop my head back up into reality to find the rest of the group is out of the water already. The guide motions to me to join them, and I begin a slow drift to the other side, still finding new angles and unexplored territory to keep me enthralled.
Back on dry land, we return to the fissure's beginning and the adventurous are invited to hurl ourselves from the banks into the freezing waters below. I leap, piercing the grey surface, adrenaline rushing as the cold stabs my skin, surfacing to scramble up the craggy banks and race up to the next-highest ledge - just because I can.
It's a harsh environment, but one providing plenty of adventure. But then Iceland is well versed at finding the silver lining in uncomfortable situations.
Perhaps best known for the colourful Northern lights, the nation wasn't really on the travel radar before the 2008 credit crunch, considered by all but the most intrepid as too out of the way and too expensive. But as the country's currency lost 44 per cent of its value in a nationwide bankruptcy, news editor around the world featured it high up in their bulletins and editions. The problem for them (or blessing in Iceland's case) was that the stock images of the nation consisted almost solely of its splendid wildlife: Icelandic horses roaming on verdant fields, geysers exploding, rugged scenery and the beautiful Blue Lagoon were beamed into the international consciousness. Suddenly, Iceland became an option for adventurous, budget-aware travellers.
And then, in 2010, Eyjafjallajokull, the volcano that caused the infamous ash cloud, erupted, bringing Europe's airways to a halt. Google searches on Iceland quickly reached into the millions, again highlighting the remarkable wilderness and adventure the nation offered (when it wasn't accidentally causing the rest of the world major problems).
Silfra is just one of those amazing natural experiences on offer and thankfully I'm soon to taste plenty more.
About an hour's drive into Thingvellir is the Gjabakkahellir cave. Here, I go from extreme clarity to extreme confusion, losing my sense of direction as my caving party creeps deeper into the lava tube. When lava leaves a volcano, it travels in channels, the tops of which cool down and harden. Inside, the molten rock continues its journey until it has nowhere to go, leaving a giant cylinder perfect for an adventure. However, it's not as though they're unexplored; in Iceland's earlier societies, criminals were made outcasts, banished from villages, and would often seek refuge from the harsh elements inside these winding hollows.
The walls vary from silky smooth to craggily uneven depending on which part we're in, the lava's speed as it cooled dictating the surface. Alongside unwrinkled green, red and yellow volcanic stone (the different colours caused by varying temperatures when cooling) grow stalagmites and stalactites. We're instructed not to touch under any circumstances. Previous parties have contained light-fingered environmental vandals, their thievery a shame as the ancient souvenirs only look like melted iron sitting on a mantelpiece without a sense of perspective. Iceland's most famous caver, Bjorn Hroarsson, has mapped hundreds of the country's lava tubes, and in our cave has replaced the missing stalactite and stalagmites with replica, but they remain a sad reminder that nothing beats the real deal.
During this dank, dark descent, we're crouching for metres of slow-but-steady progress, hunched to avoid the sharp stone of the roof, hardhats earning ever more dents as we clumsily adjust to our alien surrounds. Some spots require advancement on hands and knees and others demand we lie down and roll along the jagged floor in order to continue.
At the deepest point, our guide, Gulli, instructs us to extinguish our head lanterns, to not make a sound. We perch on boulders in the absolute darkness, craning our ears. There's nothing ... and it's incredibly peaceful. Think of the last time you were awake and saw and heard nothing. In our hectic modern-day lives, it won't have been too often.
An unexpected bonus of enjoying five long minutes in the pitch black is that it makes footing more secure, but after the torches are switched back on, Gulli quickly unsteadies our nerves with his black Icelandic humour: "What would you rather have?" The way back is spent with more than a few glances over the shoulder.
The following day I'm back above ground, booting along the Reykjanes peninsula on an ATV, 50km from the centre of Reijkavik. It's no cruise, the surface underneath the fat tyres rapidly changing from solid to loose rock, from sand to clay. Wits have to be kept at all times as we race along the challenging trail, speeding to keep up with our guide, Jakob, who seems to be relishing pushing us.
After a rugged 45 minutes, he leads us down to Selatangar beach. The wind whips violently across the desolate foreshore and we take refuge in an old fishing house built from lvastones. Here, from 1151-1884, this shelter formed part of a village of fishermen and their families. From this spot, dressed in animal skin, they would brave the savage ocean in wooden boats to land the cod that would sustain them. "Viking blood," Jakob remarks. "They were tough mothers." The fishing industry is still alive today; much of the cod landed in Iceland will reach London within 24 hours, served up in our fish-and-chips.
We hoon back along the beach, opening the throttle as we smash headfirst into the stinging wind. Our ATVs are too powerful to be showed, though, even as the terrain becomes remarkably steep. We roar upwards along thin, rocky trails - it's probably someone's land, but as it's not on a map, Icelandic law dictates it's free for everyone to use - making to the highest point of our journey, the summit of the mountain Hagafell, where we take in a 360 view of the entire peninsula. The famous Blue Lagoon is on one side; the island of Elde, where 16,000 pairs of Northern Gannets spend their days, and the fishing village of Grindavik on the other.
Such a challenging landscape demands closer inspection, so 24 hours later, I take in the peninsula by jeep with the extremely knowledgeable Clint, a non-stop fountain of information about the area and the country itself. His family have been here for generations, farming before the economy collapsed and diversifying into tourism when the global financial crisis hit.
The day is spent in awe of the power of Mother Nature s we explore kilometres of lava fields, walking through rock-strewn landscapes to stand at the precise points of eruption, Clint reassuring another isn't due for a while yet. Iceland has 30 active volcano systems, 13 of which have exploded since settlement in 1874AD, making it impossible to escape the ever-looming threat of a rainfall of lava.
Further on, we stumble through the steam at the geothermal area Krysuvik, one of Iceland's high temperature areas, breathing in the rotten-egg smell of the sulphur springs as the mud bubbles menacingly at 100 degrees. And nearby the fragile nature of Iceland is highlighted at lake Kleifarvatn, which since a huge earthquake in 2000 has shrunk by 20 per cent.
To finish, I enjoy one another of the pleasant results of Iceland's uncertain underground activity, The Blue Lagoon, a natural spa producing temperatures of up to 39 degrees. its murky mineral-rich water, a combination of sulphur and silica, is a far cry from the clarity of Silfra, however it soothes the aches and bruises accumulated during three days of action.
But although the lagoon may ease the physical reminders, it's powerless to erase the memories of Iceland's adventures, which remain forever etched into my mind.