Brennisteinsfjoll is a mountainous volcanic system on the Reykjanes Peninsula with lava fields, eruptive fissures, craters and geothermal activity. Its name refers to sulphur associated with geothermal processes in the area. Brennisteinsfjoll is less accessible than many of Reykjanes' better-known geothermal attractions and is best suited to well-prepared hikers. The landscape provides a strong example of the peninsula's geology and the interaction between fissure systems, lava and geothermal heat.