Mount Rainier is an active volcano in Washington. It has previously buried sections of the surrounding river valleys in a volcanic mudflow, called a lahar. The Puyallup River Valley is at greatest risk. Tens of thousands of people live in areas that may have as little as 40 minutes to as much as three hours to move to safety once a large lahar is detected, so the system is robust, warnings are disseminated promptly and widely, and people in harm's way are taught how to respond to the warnings and take protective actions.