Over the past decade, seismologists have uncovered a previously unknown class of earthquakes. Known as slow slip earthquakes (SSEs), these events are marked by their long duration; periodic or repetitive nature; and substantial, though harmless, increases in the velocity of intersecting tectonic plates. In southern Mexico the subduction of the Cocos plate under the North American tectonic plate has been found to harbor SSEs that last for up to a year. Compared to traditional earthquakes, which occur because of a sudden lurch between two frictionally locked plates, SSEs have been traced to the deeper rock underlying this locked layer. The rock properties that lend this deeper layer to the occurrence of SSEs, however, are poorly known. Measuring how seismic waves propagate through the varying rock layers of the south Mexican fault, Song and Kim identify the properties of rock that make this particular system susceptible to slow slip earthquakes.