The town of Vík lies directly beneath the Mýrdalsjökull glacier, which itself is on top of the Katla volcano.
Katla has not erupted since 1918, and this longer than typical repose period has led to speculation that an eruption may occur soon.
An eruption of Katla could melt enough ice to trigger a jökulhlaup, an enormous flash flood, potentially large enough to obliterate the entire town.
The town's church, located high on a hill, is believed to be the only building that would survive such a flood.
Thus, the people of Vík practice periodic drills and are trained to rush to the church at the first sign of an eruption.
A jökulhlaup is a glacial outburst flood. It is an Icelandic term that has been adopted by the English language.
Reference: Wikipedia