The geoid is the equipotential surface of Earth's gravity field that approximates mean sea level. It deviates from the reference ellipsoid by ±100 m. Mountains create geoid highs — but not as large as expected because they have low-density roots (isostasy). The largest geoid anomalies (~±80 m) come from density variations deep in the mantle. GRACE satellites measure geoid changes with mm precision, tracking ice sheet loss, groundwater depletion, and sea level change.