The sign at Geysir, the original geyser.

The sign at Geysir, the original geyser.

The sign at Thingvellir. The first character is an Icelandic letter standing for "Th."

At Geysir.

Geysir, largely defunct, but with other geysers nearby.

Strokkur.

Strokkur.

Strokkur. Early in the sequence of eruption, which takes place about every five minutes, the water seems to be pumped to above ground level.

Steam is a late stage in the eruption of Strokkur.

Strokkur beginning to erupt.

Strokkur, continuing to erupt.

Strokkur erupting with very hot water.

Steam is a late stage in the eruption of Strokkur and other geysers.

Strokkur erupting.

Strokkur erupting.

Strokkur erupting.

The end of the eruption of Strokkur.

A collapsed volcanic caldera.

Einar Gudmundsson and Thorsteinn Saemundsson, both astronomers at the University of Iceland.

Snaevarr Gundmundsson, an amateur astronomer, and Thorsteinn Saemundsson, of the University of Iceland.