Wow, I don't know how many times I've thought about this phenomena: when you're driving on a dirt road and you see ripples in the surface. I had always understood this had to do with the natural frequency of suspensions, but it turns out it's a property of the material itself. Some excellent research has shown that this effect arises regardless of the speed, weight, or even the shape of the object in contact with the ground. I'm always inspired by basic research like this...