An active volcano, Mount Rainier last erupted well over 100 years ago. While we can't say how many times Rainier has erupted over the course of its existence, we do expect it to erupt again someday--the only questions a matter of when, and to what degree.
A Mount Rainier eruption will place the Puyallup valley at risk of catastrophe from a lahar, which is a volcanic mudflow that can reach 100 feet in height and travel 45 to 50 miles per hour. According to the United States Geological Survey (USGS), "Lahars look and behave like flowing concrete, and they destroy or bury most manmade structures in their paths."
Should Mount Rainier erupt, everyone in the valley will need to avoid the oncoming lahar by evacuating to higher ground. It's for this reason that the lahar warning siren system is tested on the first Monday of each month, and it's also why we provide a map showing recommended Volcano Evacuation Routes.