As the water level rises, isolated lakes (a) link up to form a continuous ocean (a→b→c). Fish in (a) are confined; in (c) they can roam far and wide. Actually, the contour lines here represent lines of constant electric potential within a noncrystalline semiconductor, rather than lines of constant altitude in some Earthly terrain. And the issue is not the motion of fish but the motion of electrons. The (a)-to-(c) progression corresponds to an insulator-to-metal transition.