The GEOID is a smooth, irregular spheroid formed by the imaginary surface of mean sea level extended beneath the land surface.
The shape of the GEOID is influenced by the forces of gravity which makes the surface rise beneath land masses and fall beneath oceans.
Thanks, very easy to understand now but when you saying "mean sea level extented beneath the land surface", can you elaborate more on that please.
Under a land mass the imaginary surface is attracted upward by the mass of the land above.
It is the level of the water surface in an imaginary, very deep well influenced only by the gravity of the land mass surrounding.