Vulcan Askja is a large basaltic complex building solid Dyngjufjöll. It consists of several eruption centers and three caldera,the largest one has a maximum diameter of 8 km.
Volcanic activity began about 10,000 years before. The eruption in 1875 (one of the largest in Iceland - erupted was around 30 million. m3 of lava 180 million. m3of dust and ash, causing deaths of livestock and the consequent great wave of emigration to Iceland) has developed a newer, 4.5 km wide caldera now covered by water and known as the Lake Öskjuvatn.
The area is attractive to tourists, in some lakes, the water is warm enough to allow swimming in the open air in the polar podmienakch. In the nearby mountains and ran training astronauts for the Apollo program.
(° source Wikipedia)