Looking inside the Earth requires a signal that can penetrate rocks, minerals and other opaque material. Seismic waves represent such a signal. By recording them with a seismometer, researchers can draw conclusions from the recorded data about the state of the subsurface through which the waves have passed. Knowledge of subsurface stress or strain fluctuations is just as important for safety in construction and mining, for example, as it is for monitoring geological processes in volcanoes and fault zones. Now, Christoph Sens-Schönfelder from the GFZ German Research Centre for Geosciences in Potsdam and Tom Eulenfeld from the University of Jena have been able to show that the seismic waves excited by the surf, together with the effect of the Earth’s tides on the subsoil, can be used to better understand the properties of the Earth.