Þrengslaborgir, a very impressive row of craters, north of the mountain Bláfjall, from which lava, known as Laxárhraun, flowed over the centre of the Mývatn district continued down Lax­ár­dalur valley and north along Aðaldalur, covering a total area of 170 km2. Believed to be 2,000 years old and Mývatn in its present form of a similar age.