Some 4,500 people live on Heimaey, a 13-square-kilometer (five-square-mile) island off the southern coast of Iceland. It's mentioned in some medieval Icelandic sagas and was attacked in a Turkish raid in the 17th century, but events that took place on January 23, 1973, really put Heimaey on the map. On that day, the island’s inhabitants woke up to an earthquake and plumes of ash from the volcano Eldfell, followed by lava flows. Most residents were evacuated, while a few remained to defend the harbor by spraying ocean water on the hot molten lava. When the eruption subsided almost six months later, the island had grown by 2.2 square kilometers (0.9 square miles) and half of the town had been destroyed—but the harbor had been saved. Most of the island’s residents returned, and in the years since Heimaey has become known as a dramatic microcosm of Iceland’s remarkable geological activity. The 1973 eruption is the focus of the fascinating Eldheimar museum; among the exhibits is a house that, as in Pompeii, was engulfed in ash. A circumnavigation of the island offers yet another perspective on Heimaey’s remarkable geological history, and its present.