One of the most explosive volcanoes in U.S. history began its eruption with a trickle, not a blast. Mount St. Helens' gas-laden magma oozed into the cone before the mountain finally erupted in 1980.

Deep beneath one of the United States’ most closely watched volcanoes, researchers see not only a source of risk but a vast reservoir of clean, constant heat. The idea is as audacious as it sounds: ...