Even a seemingly calm ocean can have a lot going on beneath the surface. Many layers of water at different temperatures and salinities make up the ocean. Both of those variables affect density, and one stable orientation for the layers is with lighter layers sitting atop denser ones. Any motion underwater can disturb the interface between those two layers, creating internal waves like the ones in this demo. In the actual ocean, these internal waves can be enormous - 800 meters or more in height! In regions like the Strait of Gibraltar where flowing tides encounter underwater topography, large internal waves are a daily occurrence. Internal waves can also show up in the atmosphere and are sometimes visible as long striped clouds. (Video and image credit: Cal Poly)