The high-temperature geothermal area Gunnuhver (named after Guðrún, an old witch from a fairy tale) is located almost exactly at the southwest corner of Iceland near the city of Grindavík. It is part of the Reykjanes volcanic system, which is continuing into the Atlantic Ocean on the so-called Reykjanes ridge.
Today only fumaroles and mud pots can bee seen on location, whereas in the past Gunnuhver had also been home of geysers. The earthquake of 1918 formed a powerful geyser with a bowl of approximately 5 m (16 feet) across, named "Hverinn 1918", located around 200 m west of the present Gunnuhver mud crater. Even in 1927 it showed eruptions between 3 - 6 m (10 - 20 feet) at an interval of 15 minutes, but declined later on. Re-energized by an earthquake in September 1967 it used to erupt up to 12 m (40 feet) high. 1983 a nearby borehole for steam exploitation terminated its existence and only a mossy depression is left. Helgi Torfason and Kristján Jónasson provided in their publication Mat á verndargildi jarðminja á háhitasvæðum, 2006 a description of Hverinn 1918, its position on a map, and also a photo of its large, water filled bowl. A chemical analysis of its water resulted in the finding that Hverinn 1918 was a true saltwater geyser, fed subterranean by the sea.
After commissioning of the power plant in 2006 the geothermal surface activity at Gunnuhver increased dramatically. Enhanced exploitation reduces the pressure in the subterranean thermal system and thus more steam is generated. Northeast of Kísilhóll and 50 m north of the most intense steam vent activity in 2004 the large Gunnuhver crater started to grow since 2007, ejecting boiling mud more than 5 m high. From 2008 until 2010, the area was partly closed off by the civil defence due to the hazardous eruptions, the creation of new mud pots on the trail in the southern edge, and destruction of a section of the boardwalk. A very similar event took place in September 2014, when the area had to be closed for one day because a steam explosion destroyed parts of the wooden viewing platform at Kísilhóll. Directly in front of it a new mud geyser had evolved, showing up to 10 m high eruptions at intervals between 4 and 10 minutes. Its spouting acitvity persisted with decreasing vigour into 2015.
On road 425 you have to pay attention not to miss the sign at the unpaved access road to Gunnuhver. From the small parking lot a loop trail leads to the main attraction, the big mud crater also called Gunnuhver. Center of this area is Kísilhóll, a hill formed of silica. Hence all related thermal features are presented here as part of the Kísilhóll Group. To the northwest a plain adjoins, covered with further thermal features and bordered farther in the northwest by a reinforced collecting basin, which is part of the Gunnuhver power plant. Thermal features in this area are related to the Power Plant Basin Group.