A gravity hill, also known as a magnetic hill and sometimes a mystery hill, a mystery spot, or a gravity road, is a place where the layout of the surrounding land produces the optical illusion that a very slight downhill slope appears to be an uphill slope. Thus, a car left out of gear will appear to be rolling uphill against gravity. There are hundreds of recognised gravity hills around the world.
The slope of gravity hills is an optical illusion, although sites are often accompanied by claims that magnetic or even supernatural forces are at work. The most important factor contributing to the illusion is a completely or mostly obstructed horizon; without a horizon, judging the slope of a surface is difficult as a reliable reference is missing. Objects one would normally assume to be more or less perpendicular to the ground (such as trees) may actually be leaning, offsetting the visual reference.
The illusion is similar to the Ames room, in which balls can also appear to roll against gravity.