Observations show that the seismic normal modes of the Earth at frequencies near 10mHz are excited in the absence of large earthquakes to a nearly constant level. This background level of excitation has been called "the hum of the Earth", and is equivalent to the maximum excitation from a magnitude 5.75 earthquake. Its origin is debated, with most literature attributing the forcing to atmospheric turbulence analogous to the forcing of stellar oscillations by solar turbulence. Some reports also predict turbulence excites the planetary modes of Mars to detectable levels. However recent observations on Earth suggest instead a predominant source under the oceans. I will show that turbulence is a very weak source and instead it is interacting ocean waves over the shallow continental shelves that drive the "hum of the Earth". Ocean waves couple into seismic waves through the quadratic nonlinearity of the surface boundary condition, which couples pairs of slowly propagating ocean waves of similar frequency to a high phase velocity component at approximately double the frequency. This is the process by which ocean waves generate the prominent "microseism peak" that dominates the seismic spectrum near 140 mHz, but at hum frequencies, the mechanism differs significantly in frequency and depth dependence. A calculation of the coupling between ocean waves and seismic modes reproduces the seismic spectrum observed.