The fissure is a 63-metre-deep rift between the Eurasian and North American tectonic plates, which are, very slowly, separating. They gap between them grows 2cm wider a year, causing a tension that erupts into earthquakes roughly every decade. The Silfra fissure crack­ed wide open in 1789, during one such earthquake. It broke into an underwater spring and filled with water – meltwater from Langjökull, Iceland’s second largest glacier, to be precise. Taking up to 100 years to seep through porous volcanic rock, the water that feeds this fissure is pure and clear.

With my shoe drying in the perpetual summer sun, I plunge my water bottle into the fissure and take several swigs. The liquid is crisp and the minerals give it a rugged, healthy flavour.

Despite water temperatures that hover a few degrees above freezing, Silfra, with underwater visibility of about 80 metres, is a popular spot for divers and snorkellers, who are drawn to its ethereal caves and neon algae.

From Iceland’s capital, Reykjavik, the fissure is an hour’s drive east, deep into desolate Thingvellir, with not a tree, a bird or even a clump of thistles in sight.

In the car park, not far from the snorkelling site, a complete mobile snorkelling and diving operation is being run out of several vans equipped with racks of rubber suits and crates of diving masks.

Our guide tugs and shoves us into rubber drysuits that, while looking like costumes from a 1970s pornographic movie, will keep us dry and protect against the extreme temperatures. He goes through some safety rules, extolling us to avoid following the current into the large lake beyond as it would be an arduous trek back to the van after we’d been fished out.

We galumph in our heavy suits down a pebble path to a metal platform and steps that descend between the rocky walls of the rift. It is eerily silent. There are hardly any trees in Iceland – most have been felled for firewood – so there is no birdsong, just an expanse of lichen-covered moonscape and a harsh blue sky, reflected in the fissure’s water.

We flop in. The pressure is heavy on my suit but it is strange to feel so dry in water. I float forward, weightless, looking down at rocky walls that tumble into the depths of the Earth. Growing accustomed to the strange sensation of extreme cold on my cheeks and lips – the only parts of my body in contact with the water – I begin to enjoy gliding through the alien landscape. Rainbows ripple on the rocks below; the lack of fish and other signs of life contributes to the astral atmosphere.

Following the guide, we paddle to “the cathedral”, the deepest point of the fissure and popular with freedivers.

I exhale as much air as possible and plunge downwards, the vaulting sides of the fissure rising on either side. The clarity of the water makes it feel as though I am flying in a blue-misted sky. Neon algae spreads in tendrils from weird rock formations and boulders that have fallen in an arrange­ment that suggests ancient ruins. My body craving oxygen, I have to bob back to the surface.

I dive again, fighting the buoyancy of the dry suit to get more depth. My flippers propel me to one side of the blue-gloomy cathedral, where I touch the American continent, and then to the other, where I tag the European one. The tension between these two primordial plates, one support­ing the Alps, the other the Rockies, is almost palpable.

We swim towards a circular lagoon where the sub­merged rocks are shrouded with fungus, the last stop on the tour. Over on one side is another, smaller fissure, with caves at the bottom into which diving instructors fear to swim, on account of falling rocks.

My face is numb and my body chilled, but I am warmed by a sense of having seen something special. After gliding around the light-dappled lagoon, I follow the rest of my group up another set of metal steps and back onto land.

As we walk to the car park, to remove our suits and sip a hot chocolate, I peer around; volcanic rock looking like burned toast extends as far as the eye can see.

The edge of the North American tectonic plate rises like a cliff to my right and I wonder, as it continues to rub shoulders with the Eurasian plate to my left, how long it will be before this land again balloons with lava, turning the Silfra fissure into a bowl of steam.

Getting there

Finnair offers connecting flights between Hong Kong and Reykjavik, via Helsinki.