 So I've drawn here two layers of our homogenous middle part, and each of these layers has a transmitting velocity. Let's say that each of them is a little slower than the layer below it, like this. Now let's say we've got an earthquake up here, and when the ray pass comes down like this, it hits this layer. And since the lower layer is a little faster, you remember this, right? It's going to refract away from the normal, so it's going to go like that. And then it's going to do that same thing again because the third layer is even a little faster. So now it hits the underside of the layer and refracts back toward the normal and up. And this can be generalized to any sort of ray path that comes from this event. So here's another one, and it even works if you have some ray paths that don't ever get as deep as that bottom layer. So here's an example of some ray paths that go through this layered media, and remember the difference between what this looks like and what the ray paths looked like in the earlier sketch where the Earth was completely homogenous.