Buried in the depths of the oceans, over a million enormous underwater volcanoes puncture the seabed. Scientists have recorded their eruptions with some surprising results. Ahyi—in the Northern Mariana Islands in the Pacific Ocean—erupted for two weeks in 2014. Experts compared the sound of its eruptions to gunshots. Bogoslof, a shallow volcano in the Aleutian Islands near Alaska, sounded completely different when it erupted in 2016 and 2017. Vibrations can travel for thousands of miles before dissipating. It is very difficult to detect early signs of submarine eruptions, which can produce blankets of floating rocks, damaging boats and clogging harbors. Recording their sounds could help experts respond more quickly.