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A lahar from the 1982 eruption of Galunggung.

Mailboxes caught in a mudflow following the May 1980 Mount St. Helens volcanic eruption.

A mudflow or mudslide occurs when mud travels down a slope very quickly. Mudflows, which are like giant moving mud pies, happen when lots of water mixes with soil and rock. The water makes the slippery mass of mud flow quickly down. Mudflows happen most in mountainous places where a long dry season is followed by heavy rains.