Lava tube, Iceland
Inside a lava tube at Raufarholshellir. Snow still lying in July!
The geology of Iceland is unique and of particular interest to geologists. Iceland lies on the geologic rift between the Eurasian plate and the North American plate. It also lies above a hotspot, the Iceland plume, which is believed to have caused the formation of Iceland itself. The result is an island of volcanism and geothermal phenomena such as geysers.
More information at: en.wikipedia.org/wiki/Geology_of_Iceland