The geysir Strokkur, just as it is about to have an eruption. Between eruptions, the water surface heaves and sinks, and you can never be quite sure when it is about to burst. Finally, the water forms a big bubble that burst into a pillar of steam. Strokkur's outbursts can reach a height of 20 m, but the original Geysir, when it was active could burst as high as 80 m.