One of the top five dive sites in the world, Silfra is a lava fissure filled over with glacial water, which is so pristine that the visibility is 120 km deep. The water takes 300 years to reach the pool from a glacier, and as it filters through volcanic soil and rock, it purifies. It's so pure, you can drink it as you swim through it. To snorkel the fissure—which exists because Iceland is where the North American and Eurasian tectonic plates separate—you have to wear a teddy bear suit under an extremely elaborate dry suit, plus neoprene over your head and hands, since the water is 3 degrees Celsius. The dive is worth the uncomfortable gear: you'll never see a color blue like that anywhere else, and the fact that you float over the ends of the earth is pretty cool.