A gravity hill, also known as a magnetic hill (and sometimes a mystery hill 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. There are hundreds of gravity hill locations around the world. The slope of gravity hills is an optical illusion, although tour guides may claim natural 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…