In a country full of fascinating geological features, one is famous among divers: The Silfra Crack. Located in Þingvellir National Park, Silfra is actually a fault between the North American and Eurasian tectonic plates. It is the only place in the world to dive or snorkel directly in the fault between two continents and at its narrowest place scuba divers can reach side to side and place a hand on each continent. Underwater visibility exceeds 300 feet, giving you the sensation of swimming through air. The dive is very colorful: intensely turquoise blue water, neon green ‘troll hair’ algae, and jagged, mustard yellow rocks. Although the underwater colors look tropical in brightness, tropical it is not: The 35° F degree glacial melt water takes 30 to 100 years to filter through porous underground lava from the Langjökull ice cap into The Crack.

KAREN STRAUS Iceland