On 11 March 2011, when a 9.0 magnitude earthquake triggered a catastrophic tsunami off the northern coast of Japan, a network of GPS receivers present near the epicenter of the earthquake detected perturbations in the total electron content (TEC) in the ionosphere, traveling as gravity waves. For several years, scientists have observed that because of coupling between Earth's atmosphere and the ionosphere, large seismic events produce disturbances in the ionosphere. However, GPS receivers are not always present near earthquake epicenters, some large earthquakes are not associated with tsunamis, and most earthquakes (and tsunamis) are not large enough to create a detectable signal in the ionosphere. However, the earthquake in Japan occurred close to one of the densest networks of GPS receivers in the world: Japan's GEONET network, which made ionospheric TEC measurements in the vicinity of the epicenter before, during, and after the tsunami moved through the region. Galvan et al. compared GPS-based TEC measurements with models of sea surface height of the tsunami wave and Earthionosphere coupled models to assess for the first time what really drives the gravity waves in the ionosphere: the earthquake or the tsunami?