In between North American and Eurasian tectonic plates, Iceland’s Silfra fissure is one of the world’s most famous dive sites, popular with tourists who venture into its icy waters. Visibility underwater can exceed 100 metres (330 feet) and the spectacle of light and darkness is hypnotic. In the heart of Thingvellir National Park, on the edge of one of Iceland’s largest lakes, corridors of submerged rocks form deep cavities in between the two continents, which move away from each other by about two centimetres every year.