Page "Geoid" Paragraph 4

If the ocean surface were isopycnic ( of constant density ) and undisturbed by tides, currents, or weather, it would closely approximate the geoid.

If the continental land masses were criss-crossed by a series of tunnels or canals, the sea level in these canals would also very nearly coincide with the geoid.

In reality the geoid does not have a physical meaning under the continents, but geodesists are able to derive the heights of continental points above this imaginary, yet physically defined, surface by a technique called spirit leveling.