Mount Rainier is one of the largest and most dangerous volcanoes in the United States. The danger isn't an eruption, but something more insidious. The problem is that the mountain is rotten inside, and could collapse at any time.

It dominates the landscape and rises nearly three miles higher than the lowlands to the west, where millions of people live 60 miles away in the cities of Tacoma and Seattle, Washington. The threat to the local population isn't fire, but mud and ice.

Mount Rainier has an active hydrothermal system, which acts like an acidic sauna that essentially steams the mountain's rocky interior into soft, gooey clay. The rock eventually becomes so weak that it can collapse under it own weight. There are also 25 glaciers on the mountain, covering some 36 square miles with an average depth of 100 feet of ice. This ice does its share of damage, freezing and expanding, slowly eroding the volcanic rocks, and dripping melted water into the acidic interior.

Looking at Mount Rainier, you might think there is a large chunk missing from the top. You would be right. Apparently, some 5,600 years ago, the summit collapsed and turned into one of the biggest mudflows in history. This lahar, called the Osceola mudflow, was so big and fluid that it traveled at least 100 miles before stopping in Puget Sound. An ancient forest of vast trees -- some ten to fifteen feet wide -- covered the valley floors, but did little to slow the powerful flow.

There is evidence of at least 60 lahars over the last 10,000 years. The last major lahar occurred about 500 years ago, when a large chunk of the volcano collapsed and triggered mudflows that inundated the river valleys below, leaving deposits that are 30 feet thick in places. Some lahars may begin as surges of water melted during an eruption. But there is also evidence that lahars can begin with a landslide or a crumbling rock face, without volcanic prompting.


More than 150,000 people currently live in communities built on top of old mudflows, including a large part of the city of Tacoma. Despite the risks, the valleys beneath Mount Rainier remain one of the most affordable and attractive in the region, and the population has been increasing significantly. The eruption of Mount Saint Helens in 1980, when a huge lahar washed down the volcano after the initial eruption, raised awareness of the dangers of lahars. And towns close to Mount Rainier instituted evacuation drills for school children and began drawing up large scale disaster plans.

The mountain is being closely monitored. Pierce County, the county next to the weakest parts of the mountain, installed the world's first automated lahar early warning system in 1998. The system, created with the help of the United States Geological Survey, includes ten acoustic monitoring stations linked by radio to a computer that evaluates the raw data. Each station contains a sophisticated sensor and a radio transmitter, set in a 25 gallon drum. These drums are placed in the flood plains of the Puyallup and Carbon River valleys, just below the mountain, and about 25 km upstream from the little valley town of Orting.

The sensors are calibrated to tell the difference between a lahar, volcanic activity, an earthquake, and a herd of deer. Two of the stations are set in the likely path of the lahar, and are designed with "deadman switches." This means they will fall silent as they are swept away -- initiating the most urgent of warnings.

Geologists estimate that a lahar could slip down the mountain and arrive at the town of Orting in less than an hour. Or it might take just 30 minutes. Orting, with a population of 3,300, has three sirens connected to the early warning system. If the sirens go off, people are supposed to abandon their homes and climb to higher ground, following preplanned evacuation routes.

A large lahar, traveling at 30 miles an hour, would quickly sweep over Orting and continue down the Puyallup Valley, towards more densely populated areas. The towns of Sumner, Ashford, Elbe, Packwood, Randle, Greenwater, and parts of Puyallup stand in the lahar's most likely path, and might have an extra 30 or 40 minutes to complete their evacuations. Parts of Tacoma, Buckley, Enumclaw, and to a lesser extent, South Prairie, Carbonado, and Wilkeson, could also be hit as the lahar continued onwards towards the lowlands of Puget Sound. In all, 30,000 Puyallup River Valley residents could be in direct danger, along with 100,000 people living in the mountain's six other valleys.