The high-temperature region around the triple volcano system Hengill-Hrómundartindur-Grændalur in Southwest-Iceland encompasses seven geothermal areas:

• Hellisheiði (south of Mt. Hengill, site of a geothermal power plant)
• Hverahlíð (west of Hveragerði, adjoining Hellisheiði, site of a geothermal power plant)
• Nesjavellir (north of Mt. Hengill, site of a geothermal power plant)
• Bitra (southeast of Mt. Hengill, site of a geothermal power plant)
• Ölkelduháls (around Hrómundartindur volcano, east of Mt. Hengill)
• Grændalur (also called Grænsdalur or Grensdalur, around Grændalur volcano, north of Hveragerði)
• Hveragerði

Impressive hot spring fields can be found at Nesjavellir, at Innstidalur between Mt. Hengill and Mt. Skarðsmýrarfjall, at Klambragil valley south of Mt. Ölkelduhnjúkur, and at Ölkelduháls valley. In contrast to the still active volcanoes Hengill and Hrómundartindur, the Grændalur volcano is extinct and deeply eroded. Nevertheless, the valley Grændalur at the site of the former volcano is still home of hot springs and, at least true for the past, also geysers.

The geothermal area at Hveragerði in the municipality Ölfuss comprises several hot spring and geyser sites, too. One of them is Hveragarðurinn, the geothermal park in the town center of Hveragerði. The most infamous hot spring there is Manndrápshver (man killing hot spring), which got its name in 1906 after a resident of Hveragerði fell into the spring at night and was scalded to death. This fatality gave rise to the installation of street lights in the village. Another remarkable story is associated with the spring Önnuhver (busy hot spring). For many years residents had thrown their gargabe into the spring, when suddenly during an earthquake in 1947 the dormant geyser came back to life and returned all the trash by tremendous eruptions at the peoples feet. Therefore, it is also called Ruslahver (garbage hot spring). A second geyser in the park was Dynkur (blowing spring). Currently it is acting as a steam vent, but in the past it was ejecting a fine spray of water during the eruption. Further the blue pool Bláhver (blue hot spring) and the spring Gróuhver are part of the setting. Also located in the settled area of Hveragerði are Bakkahver (riverbank hot spring), a utilized hot spring covered by a pyramidal cistern roof, and Sandhólshver (sandhill hot spring) close to the church.

In the northern part of Hveragerði, across the river Varmá from the Guesthouse Frost og Funi (Frost and Fire), the now-extinct geyser Baðstofuhver (bathroom hot spring) was to be found. According to old reports prior to 1800 its eruptions out of a funnel shaped vent reached between 7 and 15 m (23 - 50 feet) height. The chronicle Descriptio Ölfuss hrepps from 1703 delivers the observations of Hálfdán Jónsson: "The vigorously boiling Baðstofuhver is located in a cavity or fissure of approximately 3.6 m (12 feet) width in the east [of Hveragerði] near Varmá river, north of Reykjahver. It is beautiful to watch it jetting boiling water and a lot of steam high into the air. During the play water discharges into the river, but some of it gets reabsorbed by the vent after the play has finished." E. Henderson, a Scotsman who visited the spring in 1815, saw it erupting to a height of 3 - 6 m (10 - 20 feet) for a duration of 10 minutes (E. Henderson, Iceland: the journal of a residence in that country during the years 1814 and 1815). In 1941 the Swiss geologist Richard A. Sonder described Baðstofuhver in his publication Studien über heisse Quellen und Tektonik in Island as a nearly permanently spouting, boiling spring, but provided no information about the height of its play. Close to Baðstofuhver six further hot springs were located including one small geyser, which showed an interval of 7 to 15 minutes in the 1930s.

Several locations with thermal features are extending along the floors and slopes of valleys north of Hveragerði. Leaving the town to the north, the road runs east of Mt. Reykjafjall. On the left side you come across the dormant geyser Grýla (name of a horrifying giantess from the mountains). A typo somewhen in the past led to the synonym Grýta, which still is in use, too. It played from a fractured, medium sized slab rock, featuring a quite small round hole in its center. The vent drops 3 m (10 feet) deep straight down until a bend can be detected. Previous to the 1970s, the geyser was spouting several times a day up to 10 m (33 feet) high. Publications from the early 1960s even state an interval of less than one hour and a maximum height of 15 m (50 feet), but that seems to be questionable. For 1934, Tomas F. W. Barth (Volcanic geology, hot springs, and geysers of Iceland, Carnegie Inst. Washington Pub 587, 1950) reported thin jets up to 6 m (10 feet) at an interval of 2.5 hours. After 1970, Grýla was decreasingly active, often only if triggered by soap as shown on the picture. Nowadays it must be regarded as dormant, if not extinct. Probably this is a consequence of geothermal drilling, which started at Hveragerði in the 1940s.

Across the road from Grýla, the river Varmá flows over a small waterfall called Baulufoss. Close to Baulufoss, 20 m (65 feet) from the river side to the west, the quiet Eldhólshver (firehill hot spring) was to be found. Furthermore, round about 300 m (950 feet) south of Grýla the geyser Gosi (boy) used to eject a thin jet of water 4 m (13 feet) high every 2 hours. In 1934 the interval shortened to 10 minutes until the geyser ceased playing in 1937. For several years it was succeeded by Gosi II. Lying only 3 m (6 feet) southeast of Gosi, it was said to spout regularly 20 minutes before Grýla.

Besides natural geysers also artificial ones were reported from Hveragerði. In the 1930s the land owner Bogi Þorðarson discharged the water of two hot springs by pipes, which he had propelled horizontally through a slope into deeper sections of their plumbing systems. Every time when he opend the valve at the end of one of the pipes, the lowered water level triggered an eruption of the related spring. The two artificial geysers were called Bogi I and Bogi II.

North of Grýla a fork in the road leads right to the golf course at Gufudalur or left to Ölfusdalur and farther to Reykjadalur. Keeping right, the borehole Leppaluði (name of Grýlas husband) comes into view on the right-hand side of the road, just in front of a bridge over the river Varmá. Currently the flat concrete funnel shows no activity, but some years ago it had been constantly erupting for a long time, reaching a height of 10-12 meters. Often Leppaluði is mistaken for the geyser Grýla, although there is a distance of approximately 1 km (0,6 miles) between them.

Crossing the river, an unpaved road branches to the right. It gives access to the thermal field Reykir, which is located on the slope of Mt. Reykjafjall north of the Agricultural University of Iceland. In the past Reykir was famous for some of the largest geysers of Hveragerði, Litli Geysir, Svaði, and Spýtir, but they are all gone for decades. Some information about them and the current thermal features is given below in the section Reykir Group.

If you follow the left road at the fork, you pass Reykjakot farm and reach the Reykjadalur parking lot. The hike along the river Reykjadalsá is leading to various springs and mud pots which are given below as Rjúpnabrekkur Group (first part of the trail) and Reykjadalur Group. Prior to 1950 the Rjúpnabrekkur Group was home of the geyser Hveramoahver and of Christmas Night Hot Spring, which was formed by a tremendous steam explosion at Christmas Night in 1915 or 1916.