Some volcanoes erupt explosively, with magma rapidly expelled as a hot jet of gas. The processes by which the magma is converted into the hot jet are still poorly understood. As discussed in the Perspective by Vergniolle, recent research reported by Morrissey and Chouet in the same issue (p. 1290) may shed light on these violent events. Data obtained with a network of microbarographs, or pressure sensors, can be used to estimate the pressure of the exiting gas jet.