South Sister Volcano, Oregon: Number 6 of America's Ten Most Dangerous Volcanoes

With South Sister (pictured) ranked by the USGS as a "very high threat" volcano, the Three Sisters area is a volcanic hotbed spanning about 115 square miles (300 square kilometers) just west of Bend, Oregon.

The next major activity in the area might not be an eruption of one of the three volcanic peaks—Middle, South, and North Sister—but the start of a new volcano altogether, the Cascades Volcano Observatory's Scott said. "It could really occur almost anywhere in that broad area."

In the late 1990s and early 2000s, an area of ground west of the Middle and South Sister started to deform. Volcanologists closely monitored the 9-mile-wide (15-kilometer-wide) area they dubbed "the Bulge," since ground deformation can indicate magma moving and accumulating underground.

The Bulge, though, is now deflating. "In the end, it didn't result in an eruption," Scott said. "But it may be evidence of a process that may eventually produce one." Source