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The only place in the world where it’s possible to dive or snorkel between two tectonic plates, the Silfra fissure in Þingvellir National Park offers a rare opportunity to see geological forces at work. Silfra is a divergent fissure between the Eurasian and North American tectonic plates, which are moving apart at a rate of about 2 cm (0.8 inches) per year.

This half-day activity is led by a qualified guide and gives you a first-hand look at the enormous fissure in a lagoon filled with clear, glacial water. With great visibility and surreal surroundings, it’s a once-in-a-lifetime experience.

Meet your guide and the others joining your small group at the Silfra fissure in Þingvellir National Park. You’ll receive a safety briefing before being fitted with a dry suit, neoprene hood, gloves and snorkel equipment. It’s important to wear your own thermals underneath as you’ll spend between 30 and 45 minutes in water that hovers just a few degrees above freezing year-round.

Before you enter the water, your guide will describe what you might see and explain how the different rock formations have been created. The lagoon in which you’ll be snorkeling is filled with meltwater from the nearby Langjökull glacier, which filters through lava fields for decades before arriving in the lagoon, cleared of all impurities. The resulting water is exceptionally clear, though the temperature takes a few minutes to get used to.

Once in the water, you’ll enjoy visibility of over 100 m (328 ft) as you swim between continents. The underwater fissure is an ethereal, midnight-blue and stretches to depths of 63 m (207 ft) in places, with steep canyon walls on either side. You’ll snorkel through the widest section of the fissure, known as Silfra Hall, and take a look into its deepest section, Silfra Cathedral, as well as swimming to the shallow end of the lagoon, which is known as the Real Blue Lagoon.

Your dry suit will help to keep you buoyant, and, on bright days, you’ll see the sun’s rays cutting through the clear lagoon as you swim through the top layer of water.

Once you’ve had some time to take in the underwater sights, you’ll get out of the water and warm up with hot chocolate and cookies. Take this as an opportunity to ask your guide any questions you may have about what you’ve seen.

This is a popular tour, so there’ll be numerous other groups snorkeling at the same time, but a maximum of eight people to one guide.