Silfra is a fissure between the North American and Eurasian tectonic plates in Thingvellir National Park. The rift was formed in 1789 by the earthquakes accompanying the divergent movement of the two tectonic plates. The diving and snorkeling site at Silfra is right where the two continents meet and drift apart about 2 cm per year. Silfra is the only place in the world where you can dive or snorkel directly in a crack between two tectonic plates.
The earthquakes of 1789 opened up several fissures in the Thingvellir area, but the Silfra fissure cut into the underground spring filled with glacial meltwater from the nearby Langjökull glacier. The water is filtered through porous underground lava for 30-100 years before reaching the spring that feeds into Silfra. The water is therefore extremely pure by the time it reaches the north end of Thingvellir lake and it allows for underwater visibility of over 100 meters in Silfra. The glacial meltwater remains very cold in Silfra, but as fresh water is constantly filling the fissure, the water never freezes and remains 2°C – 4°C year round. The underwater visibility of the water in Silfra will rarely, if ever, be surpassed. Silfra is said to have the clearest water in the world; feel free to have a sip of this pristine water at any point during your dive or snorkel.
As Silfra is right at the crux between the tectontic plates, it is a very “living” dive site in that it is constantly undergoing changes, both large and small. The fissure widens incrementally, but more drastic changes to the depth profile have occurred during earthquakes in which boulders and rocks fall into the crack. This shifting of the earth creates new tunnels, caverns, and underwater terrain.
Source: dive.is