Askja represents a volcanic system with nested calderas N of Vatnajökull. A major eruption in 1875 resulted in the formation a huge caldera which was later filled by lake Öskjuvatn (right background), 11 square kilometres large and more than 200 m deep. The small geothermal lake Víti (in the foreground) fills an explosion crater that goes back to the same eruption. It has a temperature of 20-30 degrees Centigrade. At the time this photo was taken, it had a very low ph value of approx. 1.5.