Some say it's the closest thing to space walking you will ever get. Silfra is a river in Iceland so clear that if it wasn't for the bubbles, you would think you were floating in just air. The water is melted glacial water that has been filtered through the lava bed for years and no matter if it is 20°C below or above zero outside, the water temperature is always 2-4°C (just above freezing point). Because of the cold, there is no live in the water and therefore it is so clear - with visibility over 100 meters!!! And you can drink as much off the water as you like while diving (don't overdo it though) :)
Silfra is actually a crack between the continental plates, situated in the middle of Thingvellir National Park in Iceland. This is the only place in the world where you can see the continental plates seperate at the average speed of 2 cm a year. The crack is always widening, and you are invited in. This is an out of this world experience and has repeatedly been voted one of the 10 coolest diving spots on earth.
Now if you are really into diving, you can get cave dives and deep diving in Silfra as well. Check out www.dive.is for more info.