A bizarre humming sound inside the Earth finally has an explanation

Scientists studying our planet have a pretty good idea of what goes on up here at the surface. Understanding what's going on deep underground or in the depths of the Earth's oceans is another matter entirely. Without being able to constantly observe the processes happening miles beneath our feet, researchers often have to listen instead. Two years ago, many of the instruments used to listen in on the inner workings of the Earth began acting, well, odd. All over the world, some of the most sensit...