Askja, a 50 km2 caldera in the Dyngjufjöll mountains. The bottom, about 1,100 m above sea level, is mostly covered with rough lava.

In the southeast corner, in a smaller caldera is the lake, Öskjuvatn, 217 m deep, and thus the second deepest lake in Iceland. The caldera and the lake were formed in the eruption of 1875, when there was a tremendous explosion in the small crater Víti (“Hell”) on the northeastern shore of Öskjuvatn.

This eruption produced more ash than any other eruption in Iceland, covering much of eastern Iceland, so that many farms had to be abandoned.

This was a major cause of the emigrations to North America (see Road 77, Hofsós). There have been many other eruptions in Askja, the latest in 1961 when lava flowed from Öskjuop, the craters now named Vikraborgir.

In 1907 two Germans, Walther von Knebel and Max Rudloff, were lost, presumed drowned in Öskjuvatn, while doing research at Askja.