Mount Rainier Volcano, Washington: Number 3 of America's Ten Most Dangerous Volcanoes

The danger with Washington State's Mount Rainier is that it's covered by more snow and ice than all the other Cascade Range volcanoes combined, so it presents a high risk of lahars, or volcanic mudflows. "A lahar is like concrete flowing down the chute of cement mixer," said the Cascades Volcano Observatory's Scott.

In the past Mount Rainier's lahars have run down the river valley all the way to Puget Sound, a distance of more than 62 miles (100 kilometers). "That's where a lot of the [urban] development is. so that's a key hazard," Scott said.

Mount Rainier undergoes significant volcanic activity every 500 to 1,000 years, Scott said—and the volcano's last big explosion was about 500 years ago. "But right now we know the volcano is at rest." Source