Agravity hill, also known as amagnetic hill,mystery hill,mystery spot,gravity road, oranti-gravity hill, is a place where the layout of the surrounding land produces anillusion, making a slight downhill slope appear to be an uphill slope. Thus, a car left out of gear will appear to be rolling uphill againstgravity.[1]
Although the slope of gravity hills is an illusion,[2] sites are often accompanied by claims thatmagnetic orsupernatural forces are at work. The most important factor contributing to the illusion is a completely or mostly obstructedhorizon. Without a horizon, it becomes difficult for a person to judge the slope of a surface, as a reliable reference point is missing, and misleading visual cues can adversely affect thesense of balance. Objects which one would normally assume to be more or lessperpendicular to the ground, such as trees, may be leaning, offsetting the visual reference.[3]
A 2003 study looked into how the absence of a horizon can skew the perspective on gravity hills, by recreating a number of antigravity places in the lab to see how volunteers would react. In conclusion, researchers from the Universities ofPadua andPavia in Italy found that without a true horizon in sight, the human brain could be tricked by common landmarks such as trees and signs.[4]
The illusion is similar to theAmes room, in which objects can also appear to roll against gravity.
The opposite phenomenon—an uphill road that appears flat—is known inbicycle racing as a "false flat".[5]
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