By the late nineteenth century, engineers and experimental scientists generally knew how radio waves behaved, and by 1901 scientists were able to manipulate them to transmit messages across long distances. What no one could understand, however, was why radio waves followed the curvature of the Earth. Theorists puzzled over this for nearly twenty years before physicists confirmed the zig-zag theory, a solution that led to the discovery of a layer in the Earth's upper atmosphere that bounces radio waves earthward-the ionosphere. In Probing the Sky with Radio Waves, Chen-Pang Yeang documents this monumental discovery and the advances in radio ionospheric propagation research that occurred in its aftermath. Yeang illustrates how the discovery of the ionosphere transformed atmospheric science from what had been primarily an observational endeavor into an experimental science. It also gave researchers a host of new theories, experiments, and instruments with which to better understand the atmosphere's constitution, the origin of atmospheric electricity, and how the sun and geomagnetism shape the Earth's atmosphere.