Mount Etna's spectacular eruption on Monday, which caused a partial collapse of the volcano's crater, prompted a local volcanologist who witnessed the eruption to warn future visitors to the mountainous island. At around 10 a.m. on Monday, Etna exploded with its first major, violent eruption of the year: lava fountains and a column of ash and gas rose several kilometers, or miles, in the air. The event climaxed around 11:23 a.m. when the pyroclastic flow, triggered when magma mixed with snow, traveled 2 kilometers (more than a mile) to the Valle del Leone within a minute. (AP production by Paolo Santalucia)