Mount Rainier looks constant and unchanging, an impassive landmark on the horizon. But the mountain hasn't always been so quiet.

About the time Christopher Columbus was arriving in America, a torrent of mud rolled down Rainier's western flank, burying the spot where Orting stands today. Five hundred years before that, an eruption sent a mud flow down the White River Valley to what is now Auburn.

Now, with thousands of people moving within reach of Mount Rainier, scientists say they need to "listen" much more closely to the third-most dangerous volcano in the nation.

Over the next two years, the U.S. Geological Survey (USGS) plans to increase the number of earthquake monitors from five to nine, ring the mountain with eight new global-positioning (GPS) units to monitor the mountain's movements and speckle it with 21 small metal discs to gauge whether the mountain changes shape.

Mount Rainier National Park officials announced the plan Tuesday, saying the potential environmental impact of the plan will be studied before permits are issued. Park officials are now accepting public comment.

Today, scientists track the mountain's movements primarily with the five earthquake stations. That's enough to be sure they aren't blindsided by an unexpected eruption, said Cynthia Gardner, scientist in charge of the Geological Survey's Cascades Volcano Observatory in Vancouver, Wash.

But they still have significant blind spots, she said.

For example, they can't watch for subtle flexing in a volcano's surface, she said. That can be an early sign of a pending eruption or the collapse that leads to destructive mud flows. The GPS units would do that by beaming a continuous stream of data back to scientists.

The additional earthquake gauges would enable scientists to get a better picture of an earthquake, such as how deep it is. The depth can indicate whether molten rock is moving toward the surface.

In 2005, the USGS declared Mount Rainier a "significantly under-monitored" volcano. It also ranked it the country's third-most dangerous based on how big an eruption could be and how much damage it could cause. The top two are Mount St. Helens and Hawaii's Kilauea.

The present system's limitations were made apparent in October, when a 4.5-magnitude earthquake struck Rainier's eastern edge, said Seth Moran, a seismologist at the Vancouver observatory.

Earthquake monitors showed that aftershocks could be edging closer to the mountain's center, a possible sign of something bigger to come.

It turned out to be nothing - a false clue created by gaps in the network, Moran said. One of the proposed new monitors would sit right near the epicenter of that quake.

The proposed improvements were welcomed by John Vidale, director of the University of Washington's Pacific Northwest Seismic Network, which monitors earthquakes in Washington and Oregon, and has worked with the USGS on the plan.

"The proposed upgrade will help a lot," he said. "But in the end we'd like to get twice as much instrumentation up there."

Top 10 most dangerous U.S. volcanoes

Based on U.S. Geological Survey ratings of size and potential damage of an eruption.

1. Kilauea, Hawaii

2. Mount St. Helens

3. Mount Rainier

4. Mount Hood, Ore.

5. Mount Shasta, Calif.

6. South Sister, Ore.

7. Lassen Volcanic Center, Calif.

8. Mauna Loa, Hawaii

9. Redoubt, Alaska

10. Crater Lake, Ore.