Hakaudalur Valley is one of the world’s iconic geothermal areas, primarily because it’s home to the the original geyser, Geysir. Geysers are actually a type of spring–just a spring which sometimes violently ejects water and steam.
Geysir was the first geyser to be described in printed sources, and so gave its name to all similar phenomena. While it’s been active for 10,000 years, Geysir can go a long time between eruptions, or even cease entirely for periods, but can be spectacular when it erupts.
However, the nearby geyser Strokkur (‘The Churn’) is less impressive, but far more reliable, erupting every few minutes. Here it is between eruptions:
And during an eruption (apologies for the touristy photos: to get truly good photos of this area, I think you have to go in the middle of the ‘night’ during high summer):