The geoid is theoretical only. You can't see it, touch it or even dig down to find it. Simply put, the geoid is the natural extension of the mean sea level surface under the landmass. We could illustrate this idea by digging an imaginary trench across the country linking the Atlantic and Pacific oceans. If we allowed the trench to fill with seawater, the surface of the water in the trench would represent the geoid. Not a bad way to imagine the geoid, but in reality not something we could easily do.
Digging an imaginary trench