 As a given mechanism, it's already in operation, but it has to start somewhere. Did it start with the trenches or did it start with the ridges? The new material coming up or the old material sort of going down and dragging the plates with them? Right. Well, I think it actually began with the heat, the convection driving up and creating the ridges. Well, then the Earth's lithosphere, the upper mantle and the crust, can accept an awful lot of compression. And through...