Árhver (and nearby Vellir) is a geological feature which consists of six to eight vents on small sand flats, some slightly elevated but most just beneath the surface of the water.
Lucky visitors might be able to see it erupt, shooting water up to 1.5 metres (5 ft) into the air, although the activity has been slowing down in recent years. However, the ever-changing landscape of Iceland means there are no guarantees it will stop. In fact, after some major earthquakes in 1896, it started to erupt at heights of over 10 metres (32 ft), so no one knows what more seismic activity could cause it to do.
Explore this area of Iceland on a self drive tour.
Photo above from Wikimedia, Creative Commons, by Reykholt. No edits made.