By sending seismic signals, really sound waves, from the surface to great depths, and recording reflections from there. In a later slide, we'll find out more about how this technique works. This is how LITHOPROBE's scientists obtained the deep reflection data from which they could construct this "seismic cross section."
These visits to the Pacific "active" and Atlantic "passive" continental margins have given us some idea of the dynamics of plate tectonics.

Now, let's move into the heartland of the continent, onto -- and into the inner space of -- the Canadian Shield.