Mount Rainier (14,410 feet/4,392 meters) has not erupted since the first half of the 19th century, and the mountain hasnt experienced a truly large eruption for about 1,000 years.

While experts say there are currently no signs of imminent danger from volcanic activity, the impact associated with an eruption or other volcanic event on the mountain is far greater today than in years past.

William Scott, a geologist with the United States Geologic Survey (USGS) in Vancouver, Washington, told National Geographics Ultimate Explorer host Lisa Ling how the potential problem has grown more dangerous: "We have no sign that the volcano is doing anything different or is any more likely to erupt than it was 10 or 20 or 30 years ago. The issue is that people have moved in much, much closer."

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