Geysir (sometimes known as The Great Geysir), in the Haukadalur valley, Iceland, is the oldest known geyser and one of the world's most impressive. Eruptions may reach up to 70 to 80 m but are infrequent. It is most active after earthquakes but can stop altogether for years at a time.

The nearby geyser Strokkur erupts much more frequently than Geysir, erupting to heights of up to 20 metres every five minutes or so. Strokkur's activity has also been affected by earthquakes, although to a lesser extent than the Great Geysir. There are around thirty much smaller geysers and hot pools in the area, including one called Litli Geysir ('Little Geysir').