Mount Rainier is the tallest mountain in the state of Washington, and in the Cascades Mountain Range, and is an active stratovoclano.
Things that melt: Glaciers on top of huge volcanoes
The most massive stratovolcano of the Cascades Mountain Range is located in close proximity to the Seattle and Tacoma metropolitan areas. Due to potential catastrophic damage in the case of sudden melting of the mountain’s glaciers, which could also include a massive eruption, Mount Rainier is on the list of the world’s most potentially dangerous volcanoes.
The volcano has been relatively quiet for over a thousand years, with only a few smaller eruptive events in recent centuries. However, the volcano is certainly still active and due for a larger eruption someday, although no one can predict when it will occur.
The last sizable eruptions known took place about 2,200 and 1,100 years ago, although they were smaller than what occurred with another Washington stratovolcano, Mount St. Helens, in 1980. The most recent small eruptive event took place in 1894.
Glacial ice on the mountain is estimated at about one cubic mile, and a major cause for concern. This quantity of ice calculates to 4.2 billion tons of ice. Imagine all of this melting quickly in response to eruptive activity, or to hot magma rising within the mountain. It would flood down the slopes, and much of it would slam into and across the nearby heavily populated areas on its way to Puget Sound.
Note: In this article I use some basic volcanology terms, which are defined in Types of Volcanoes and Volcanology Terms.
Mount Rainier towers over Tacoma, Washington. Much of this city of over 200,000 residents lies directly in the potential pathway of massive lahars, or volcanic mud and debris flows, in the event of eruptive activity from the volcano.
Mount Rainier's history shows what it can do
The stratovolcano has existed, and has been active, for at least 500,000 years. Numerous eruptions have occurred, although I would like to focus on the largest one of the past 10,000 years, which occurred in about 3,600 BC.
The event, called the Osceola Mudflow, was an eruption that caused one cubic mile (about four cubic kilometers) of material to come down the northeast side of the mountain. The debris flow, or lahar, is an indication of what could happen again sometime. 
When Mount St. Helens, located 50 miles (80 km) southwest of Mount Rainier, had its major eruption in 1980, the resulting lahar was far smaller. In fact, the amount of material that flowed down the flanks of Mount Rainier was 1,400 times the quantity.
Consider that Mount St. Helens was the largest avalanche of material humans have ever observed, and it shows that we have not at all seen what nature is capable of doing. What Mount Rainier has done is far scarier. It completely altered the surrounding terrain, as flows hundreds of feet thick ran into and across much of Puget Sound.
This eruption from 5,600 years ago is relevant today is because the quantity of material that comprised the lahar is almost exactly the same as the quantity of ice that now sits on top of the mountain in the form of glaciers. Much of this ice, plus other rock, soil, and debris, could come crashing down across Tacoma and downtown Seattle, and send tsunamis across Puget Sound.
Consider this: Zoning restrictions due to volcanic hazards are non-existent. Currently there are more than 150,000 people living below Mount Rainier, whose homes sit on old mud flows from the volcano, particularly the Osceola Mudflow. This indicates that these homes are certainly in the potential path of destruction.
Only four mountains in the contiguous 48 states have a peak with a higher elevation than Mount Rainier, and none of them are volcanoes. They are Mount Whitney in California, and three peaks in Colorado.
Mount Rainier as it is now
This massive stratovolcano is the tallest mountain in the state of Washington, and in the Cascades Mountain Range. The peak sits at 14,411 feet (4,392 meters), and the mountain towers 13,000 feet (3,960 meters) above the nearby metropolitan areas. It is simultaneously very beautiful and scary.
It is a wonderful national park with skiing, mountain climbing, camping, and many exciting activities. Climbing the mountain is considered dangerous, but not due to the eruption threat. Thousands succeed at reaching the summit every year. Most take routes that originate from Camp Muir on the southeast flank of the volcano. Only half of the attempts are successful, because adverse weather and problems related to the physical conditioning of the hikers often result in attempts having to being aborted.
As many as seven small eruptions occurred in the 19th century, although the volcano is currently being relatively quiet. Small earthquakes originating inside the mountain occur all the time (typically several per month), which will someday culminate in another eruption. When and how big are unknown.
Where I live in Southern California, we prepare for large earthquakes, which hit every so often. We’re overdue for a really big one, magnitude 7.5 or greater. There are earthquake drills in the public schools, and residents are encouraged to make items in their homes secure. There are also strict building codes.
Washington state has potential for larger earthquakes than Southern California, possibly as large as 9.0, originating with the Cascadia Subduction Zone off the coast, and they have the volcanoes as well, which will strike again someday. Compared with a human lifespan, eruptions and super-size earthquakes are few and far between. However, when one hits there will be a lot of problems despite whatever preparations have been made.
Preparation can, however, minimize the problems that will result. So volcanic activity is closely monitored, and warning systems are in place including sirens, such as might exist in coastal areas prone to tsunamis coming across the ocean due to a large earthquake. Residents take part in drills that include getting to higher ground, which would be necessary in the event of massive flooding.
A lahar flowing northwest, toward Tacoma, could reach Puget Sound within one hour of the eruption, so time is short but not so instantaneous that residents living beneath Mount Rainier would be entirely unable to react.
Mountain climbers traverse one of the glaciers upon Mount Rainier on an attempt to reach the high summit.