Seen here are a group of snorkelers at the Silfra canyon, a rift between the tectonic plates (North American and Eurasian) at Þingvellir National Park, Southern Region, Iceland.

Silfra was formed as a consequence of the two tectonic plates drifting apart. Each year, the plates drift about 2 cm farther apart, which builds up tension between the plates and the earth mass above. This tension is released through a major earthquake approximately every ten years. In these earthquakes, cracks and fissures are formed in Þingvellir. Silfra is one of the largest cracks and started with a deep cave where most of the underwater wells feed it. The site lies at the rim of the Þingvallavatn Lake. [source]