The Strokkur Geyser in Iceland, which erupts every several minutes, to the delight of many tourists. I was quite pleased with having captured the initial bubble preceding the eruption, right at the point of breaking and the spout shooting up.

Note of warning: if you ever find yourself in position to watch a geyser, make sure you stand upwind... a trick some of the people in this picture are going to learn the hard way :)
Note of warning: if you ever find yourself in position to watch a geyser, make sure you stand upwind... a trick some of the people in this picture are going to learn the hard way :)