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.