Iceland features stunning waterfalls of all sizes and descriptions, but at Seljalandsfoss, in South Iceland, you can actually walk behind the curtain of water. At the Silfra Fissure in Þingvellir National Park, you can snorkel or scuba dive in the crack between the North American and Eurasian continents. Fed by Langjökull glacier, the water in the fissure is famously clear, with exceptional v…