I'm surfing on the inside

I've always loved the idea of internal waves; the idea that gentle, rolling, and sometimes very large waves roll along, not on the surface of the sea, but deep below it.  How is that possible?  The best explanation is by analogy:  If you've ever swum in a lake in early summer, where your body was bathed in warm still surface waters but your legs were down in the icy deeper layer, then you've crossed the boundary that internal waves call home.  They travel along "density boundaries" and the most common of those is the bit where water goes from warm at the surface to cold at depths, called the thermocline; there are other types of density boundaries too, such as where fresh water overlies denser briny water.  Sometimes you can even see density boundaries near the surface; the change in density affects the way light passes through the water so you can sometime see a shimmery sort of distortion, even though the water is clear.  There's a good one shown here. The picture at right from the Institute of Hydromechanics at the University of Stuttgart shows an artificial internal wave produced as part of an experiment; they dyed the different densities of water to show it better.

The bigger the change from warm to cold in a thermocline, the bigger the density difference, with the colder water being more dense.  Really sharp thermoclines like this have some interesting properties, such as the ability to reflect some types of sonar.  In fact, Navy submarines have been known to "hide" below a good thermocline, and then be revealed for all to "see", by a passing internal wave.  The sub is below the thermocline one minute, then above it the next, exposed and vulnerable to the next sonar ping - oops. 

Internal waves can even "break", like a surf wave on a beach.  This image from Memorial University in Canada shows a model of how this happens, with the wave coming in from the left and breaking on the bottom as it gets shallower; all the while the water's surface is calm.  The internal wave doesn't look quite the same as a regular beach wave because the density difference isn't nearly as much as when you go from water to air and the internal wave pushes up against the heavy, viscous overlying water, but the principle is the same.

I don't know if its possible to somehow surf on an internal wave.  I doubt it, because the drag of the overlying water would be much more than you would experience in air, but its pretty cool to think about.  Its probably a good thing if you can't do it, because Al surfing is about the only thing scarier than Al dancing...