Mount Shasta Volcano, California: Number 5 of America's Ten Most Dangerous Volcanoes

Flanked by an interstate, California's Mount Shasta volcano looms over thousands of homes, a key to its USGS "very high threat" rating.

Around Mount Shasta an eruption's pyroclastic flow—rapid currents of superheated gas, ash, and rock caused by a volcanic explosion—as well as ash-infused mudflows could put towns and infrastructure in harm's way.

The last reported eruption was seen from the Pacific Ocean in 1786 and may not have "been such a big deal," the Cascades Volcano Observatory's Scott. "We haven't had [an eruption] since settlement by European settlers, but in the geologic sense the volcano has been quite frequently active." Source