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

Optical illusion

An optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source...

 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 due to gravity.

There are hundreds of gravity hill locations around the world. See list of magnetic hills.

In bicycle racing

Bicycle racing

Bicycle racing is a competition sport in which various types of bicycles are used. There are several categories of bicycle racing including road bicycle racing, cyclo-cross, mountain bike racing, track cycling, BMX, bike trials, and cycle speedway. Bicycle racing is recognised as an Olympic sport...

, such roads are known as "false flats."

Explanation

The slope of gravity hills is an optical illusion, although sites are often accompanied by claims that magnetic or even supernatural

Supernatural

The supernatural or is that which is not subject to the laws of nature, or more figuratively, that which is said to exist above and beyond nature...

 forces are at work.

The most important factor contributing to the illusion is a completely or mostly obstructed horizon

Horizon

The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...

; 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

Perpendicular

In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...

 to the ground (such as trees) may actually be leaning, offsetting the visual reference. The illusion is similar to the well-known Ames room

Ames room

An Ames room is a distorted room that is used to create an optical illusion. Probably influenced by the writings of Hermann Helmholtz, it was invented by American ophthalmologist Adelbert Ames, Jr...

, in which balls can also appear to roll against gravity.

External links

• Mystery Spots and Gravity Hills at About.com
  About.com
  About.com is an online source for original information and advice. It is written in English, and is aimed primarily at North Americans. It is owned by The New York Times Company....
  
  
• The Enigma Project