By Sid Perkins
A two-week cruise on an icebreaker to the top of the world last summer gave scientists a look at the aftermath of an event once thought impossible: a violent volcanic eruption on the deep-sea floor.
![Layers of volcanic ash (samples shown in inset) blanket the Arctic seafloor 4,000 meters down. The ash is evidence of an explosive eruption, long thought impossible at those depths.](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2008/06/8232.jpg?resize=300%2C212&ssl=1)
In 1999, a global network of seismic instruments detected the largest swarm of earthquakes ever to occur along the planet’s system of mid-ocean ridges, where tectonic plates spread to form new ocean crust. Several aspects of the recorded vibrations suggested that the quakes were generated by volcanic activity, says Robert A. Reves-Sohn, a geophysicist at the Woods Hole Oceanographic Institution in Massachusetts.
However, he notes, many scientists have doubted that explosive volcanism can take place at the 4,000-plus-meter depth where these quakes occurred because the immense pressure of overlying water prevents seawater from flashing into steam, a major driving force for such eruptions.