Þeistareykir is a high-temperature geothermal area with very lively surface activity. Surface manifest-ations are seen in northern Mt. Bæjarfjall extending to the east into Bóndhóll and Mt. Ketilfjall. Large mud-pools and solfataras are, for example, found west and south of Tjarnarás. In the lava west of Mt. Bæjarfjall the hot groundwater vents odourless steam which is uncontaminated by volcanic gases. The hottest and most powerful area of the geothermal system is found east of Tjarnarás. Some of the largest sulphur mines in Iceland occur at Þeistareykir, where the sulphur was unusually pure and was mined well into the nineteenth century. The last eruption in Þeistareykir occurred about 2400 years ago, but the next eruption before took place soon after the area was deglaciated.