The crater duet of Askja and Viti is among those things which you have to see to understand. Word alone cannot do justice to these places which are so extraordinary to experience - like being on the Moon!
Askja has had several known eruptions, the last one was in 1961. This eruption was reasonably small and caused no problems. However, in 1875 a major eruption took place which caused this crater to be well known. The tephra and ash from this eruption resulted in famine across Northern Iceland and Scandinavia causing the widespread death of livestock. These events also sparked Iceland's greatest emigration to North America.
Today there is a very large lake which half fills the crater. Until a few years ago this was Iceland's deepest lake at 220 meters, however, Jokulsarlon is now deeper. There is also a smaller lake situated in another small crater inside the Askja crater. The English meaning of the name Viti is "hell" and it never freezes! (Have you ever heard of Hell freezing over?). This lake is boiling in parts, whilst in other places the water is perfect for a hot bath, making it one of the most popular places to visit in the North Icelandic highlands.