One of many ice-cauldrons in Mýrdalsjökull, which are geothermally formed. Some are aligned above the eastern rim of the Katla caldera, which lurks underneath the ice-cap. In the center of the caldera, the ice is over 500 m thick. Melt water from the surface of the glacier has collected in the bowl, giving the blue colour, but that is irrelevant to the activity discussed in this article.