Geysir is a geothermal area full of hot springs. The main geyser is Geysir, which gave the name to other hot springs around the world. Often referred as the Great Geysir, we never saw it in action: looks like it is inactive. In the past, icelandic authorities tried to unclog the underground caves, but from time to time, due to frequent earthquakes, rocks would close any water passages.
Only 50m from Great Geysir, there is Strokkur. This geyser is active, it sputs every 8-10 minutes. A few seconds before it goes off, some minor changes in the water allows you turn on the camera. It surely is an amazing view, where at first a dome of water creates and then burst in the air.
This area is really crowded, since it is one of the main attractions of Iceland. Day tours often comes here, and its short distance from Reykjavik mixed to iconic views, makes it a worthy place.