Gravity Hill

A gravity hill is a location where the surrounding terrain is arranged in such a way that a slight downhill slope creates the illusion of being an uphill slope; a car placed in neutral gear on such a spot appears to roll uphill against gravity, an effect that seems to defy ordinary physics even though the true slope of the ground still runs downhill in the direction the car actually rolls. The cause of the illusion lies in visual perception rather than in any magnetic or supernatural force, and the primary factor is an obstructed horizon: when a true, level horizon line is not visible, the brain has no reliable external reference for judging which way is uphill, and misleading landmarks nearby, such as trees or signs that are themselves tilted relative to true vertical, provide false visual cues that confuse the brain's sense of balance and spatial orientation. A 2003 Italian study confirmed this mechanism experimentally by recreating antigravity locations under controlled laboratory conditions, finding that the human brain could reliably be tricked by common landmarks such as trees and signs once a proper horizon reference was removed. The gravity hill illusion parallels the Ames room illusion, in which objects also appear to defy gravity and ordinary spatial expectations, and it has a mirror-image counterpart in cycling known as a false flat, in which an uphill road misleadingly appears flat.