By comparison in most dive sites, visibility is somewhere between 10 and 30 metres. “That’s the draw for people, to be able to see water that blue,” Taz adds.

It’s not just the water quality which attracts the estimated 25,000 divers who come here every year however. Once you break beneath the surface, the geology of lake itself is incredibly dramatic. “Basically what Silfra is is a rift, it’s a crack,” explains Taz. It’s formed by the fault-line that cuts across Iceland, where the Eurasian and the North American tectonic plates meet. At various points along the fissure’s length, divers can reach out and literally touch two continents at once.

The lake is also constantly changing. “There are hundreds of earthquakes every day in Iceland,” says Taz. “I normally go there once every couple of weeks now and you’ll notice a rock’s in a different place and another lava tube or black hole has opened up, so you can see a bit further down.” The plates are moving apart as fast as an inch every year, creating new land beneath the water as they go.

As with all extreme environments you need gear that’s 100% reliable, and you need to be checking and re-checking it constantly. But according to Jonathan, the essential element to diving safely in Silfra safely is timekeeping. “The key here is timing. You’ve got the weather to contend with, you’ve got the amount of people to contend with, you’ve got other tour groups going out. Especially when the conditions are bad, when we have serious winter conditions, it’s about doing things quickly but correctly.”

It can go horribly wrong horribly quickly too. A Chinese woman in her 20s was killed in a diving accident last year, and several tourists who were snorkeling have suffered heart attacks because of the shock of the cold. However such incidents are extremely rare, thanks to the precautions and preparations put in place by experienced guides like Taz, Chris & Jonathan.

“Silfra’s actually very safe,” says Taz. “All the companies that work here work together to train new guides on what to do if there was an emergency. We have oxygen and AED’s [defibrillators] at both the entrance and exit points, and all of our staff are emergency first response instructors.”

“The key here is timing. It’s about doing things quickly but correctly.”

There are some risks that are mitigated by Silfra’s location. As well as banning cave diving The National Park authorities, who administer the site, limit divers to a maximum depth of 40 metres, so the risks of decompression sickness are negligible. But while you won’t get the bends if you get your timings wrong here, if you want to get the most out of your dive, knowing when to go is essential.

Visitors’ experiences of it can be wildly different depending on when you visit and crucially, whether you start your dive on time. “In the summer months it gets covered in troll’s hair, which basically looks like snot, long green tendrils” says Taz, chuckling. “Now it’s all dying off because it’s coming into autumn and it looks like blobs of silver.”

In winter the extreme latitude means there’s limited daylight. “You want to make sure you start your dive at the right time, so you get those beautiful rainbows. On Christmas Eve for example I got three hours of light.”

The fact that it’s open year-round and changes with the seasons is a major part of the lake’s magnetic attraction for Taz though. “Because it’s ever-changing you always find something new every time you go.” Which is why she, and countless others like her, come back time and time again to take the plunge in this truly unique corner of the planet.

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