Primary microseisms are background seismic oscillations recorded everywhere on Earth with typical frequencies 0.05 < f < 0.1 Hz. They appear to be generated by ocean waves of the same frequency f, propagating over shallow bottom topography. Previous quantitative models for the generation of primary microseisms considered wave propagation over topographic features with either large scales, equivalent to a vertical point force, or small scales matching ocean wave wavelengths, equivalent to a horizontal force. While the first requires unrealistic bottom slopes to explain measured Rayleigh wave amplitudes, the second produced Love waves and not enough Rayleigh waves. Here we show how the small scales actually produce comparable horizontal and vertical forces. For example, a realistic rough bottom over an area of 100 km(2) with depths around 15 m is enough to explain the vertical ground motion observed at a seismic station located 150 km away. Ocean waves propagating over small-scale topography is thus a plausible explanation for the observed microseisms at frequencies around 0.07 Hz. Plain Language Summary Microseisms are background oscillations of the solid Earth. Most of these oscillations are caused by ocean waves and can thus be used to study their source, the ocean waves, or the medium in which they propagate, the solid Earth. Several theories have been proposed for how ocean waves going over shallow ocean topography make microseisms in the band of periods 10 to 20 s, but they are not satisfactory because they either require unrealistic large slopes of the ocean floor or they produce a ratio of different types of seismic waves, Love and Rayleigh waves, that is too large. We thus revise these theories to show that a plausible seismic source is the propagation ocean of waves over a wavy bottom, when the bottom has wavelengths that match those of ocean wave. We particularly verify that the predicted Rayleigh wave amplitude is of the order of what is measured at a particular seismometer located in Ireland. Because the necessary details in bottom topography vary a lot between different ocean regions, the new theory suggests that the spatial distribution of seismic sources is more heterogeneous than previously thought.