Mt Hverfjall, to the east of Mývatn is a gigantic tephra crater about 140 m deep and 1,300 m diameter, a so-called Tuff Ring, created 2,500 years ago. Tephra has been carried from Hverfjall, all over the Lake Myvatn area. A landslide apparently occurred in the south part of the crater during the eruption, which accounts for the disruption of the round shape of the mountain. The rim of the crater is only accessible by two trails, from the northwest and south. It is strictly forbidden to use other routes up or down.