Plumes of volcanic ash are a well-known threat to aircraft. A similar threat occurs beneath the ocean surface: Undersea volcanoes belch out pumice that floats to the surface and then clumps together into large, sometimes more-than-a-meter-thick rafts of frothy rock. This material can clog the engines and intake valves on ships and can even drift long distances to block harbors. Now, simulations of pumice dispersal from a submarine eruption in the southwest Pacific in 2012 offer hope of advance warning of these perils. The eruption, which took place mostly over the course of a single day, emanated from the Havre undersea volcano (which lies about 800 kilometers northeast of New Zealand and whose peak is more than 700 meters below sea level). Rafts of pumice quickly covered 400 square kilometers of sea (image shown was taken 28 July 2012, about 8 days after the eruption) but later broke apart into streamers hundreds of kilometers long. When researchers fed data about ocean currents and typical winds in the region at that time of year into a program that simulates how ocean eddies tend to disperse floating objects, they successfully tracked the pumice as it spread to afflict an area twice the size of New Zealand. In the future, similar simulations could track pumice patches in near-real time, providing navigators with a weather forecast–like early warning system, the team reports online today in Nature Communications.