Silfra, water as clear as it gets

The water is Silfra is without a doubt one the (if not the) clearest and most pristine water on the planet.
The reason harks back to approximately 12,000 years ago to the end of Iceland’s last ice-age.
When the ice retreated to where Lángjökull currently stands, it left behind a lot of glacial melt water.
Around the same time an eruption occurred, creating the volcano Skjáldbreiđur and a 327km2 lava field that covered the melt water and forced it below ground.
It is this event that we have to thank for the crystal clear clarity of the water, as melt water is naturally very murky, but lava filtration is one of the best in the world.
Now, with the melt water from Lángjökull travelling 50km or so through the lava, a journey that takes between 30 and 100 years, and welling up from an underground spring directly into Silfra, we find ourselves looking at a fissure of perfectly filtered crystal clear water.

The force of the spring is strong enough that it creates a current that runs through the crack.
Although mild, the current takes particles that fall into Silfra, or might be kicked up by a diver or snorkeller, out into Lake Thingvellir, returning Silfra to her perfect clarity within minutes.
For recreational divers and snorkelers, this clarity provides a unique experience, not only for the visibility it provides, but also because at a constant temperature of 2-4 C it provides for a deliciously refreshing drink as they swim along!