Hverfjall (also known as Hverfell) is a tephra cone or tuff ring volcano in northern Iceland, to the east of Mývatn.It erupted in 2500 BP in the southern part of the Krafla fissure swarm.
We climbed the crater on a cloudy / foggy day and it really wasn't worth it because visibility was very limited at the top. So check the weather before going!