: An eruption of the Grímsvötn volcano in May 2011 was the largest eruption in Iceland in nearly 100 years, producing a 20 km ash plume. One hour before the eruption, a live stream of data from a GPS tracking station on the flank of the volcano recorded a more than half meter shift in position. The tracking station was set up by Sigrún Hreinsdóttir of the University of Iceland in Reykjavik and her colleagues. Grímsvötn is hard to monitor with traditional techniques because it is mostly buried under ice, so the GPS station was placed on an exposed area of rock. After the eruption, Hreinsdóttir's team worked to relate the size of the GPS shift with a model of the changes in the pressure inside the volcano's magma chamber. The connection was also strongly tied to the size of the ash plume. If the effect can be shown on other volcanos, the ability to monitor and predict the occurrence and size of remote eruptions would be useful not just to scientists but to emergency officials and to pilots who need to avoid ash plumes.