Not long after NASA's Cassini mission arrived at Saturn in 2004, researchers realized that the planet's rings were oscillating strangely. Instead of single waves, which are predicted by existing theory, the spacecraft revealed clusters of small waves that could be explained by the presence of gravity waves in the deepest part of the planet's interior. On rocky planets like Earth, disturbances beneath the planet's surface can move as a wave, traveling through the planet's interior and through its surface.... Eventually, interference from other traveling waves can create a standing wave pattern spanning the entire planet. The same process occurs on...