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 in the south Pacific Ocean, near the remote Solomon Islands, there’s a volcano unlike any other on earth. For one, it’s underwater; and secondly, it’s extremely active. Kavachi, it’s called. And ...