Askja is a large caldera complex in the Dyngjufjoll mountains of Iceland's central highlands north of Vatnajokull. Major volcanic activity occurred there in 1875, associated with the formation of Lake Oskjuvatn and the Viti explosion crater. The landscape is defined by calderas, lava, mountains and other evidence of powerful volcanism. Askja lies within Vatnajokull National Park and is accessible only during part of the year via highland roads. Visitors should check road, weather and national park information before travelling.