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.

The Axial Seamount – located hundreds of miles off the coast of Oregon and nearly 5,000 feet below the Pacific Ocean’s waves – erupted in April 2015, spewing a mile’s worth of lava onto the sea floor.

The explosiveness of a volcanic eruption depends on how many gas bubbles form in the magma—and when. Until now, it was thought that gas bubbles were formed primarily when the ambient pressure dropped ...