Sometimes you may find or hear of a mysterious place where objects can apparently roll uphill. This is a remarkably common illusion, which is found in numerous locations around the world. Usually it is a stretch of road in a hilly area where the level horizon is obscured. Objects such as trees and walls which normally provide visual clues to the true vertical, may be leaning slightly.
This creates an optical illusion making a slight downhill look like an uphill slope. Objects may appear to roll uphill. Sometimes rivers even seem to flow against gravity. Spots where the illusion is especially powerful often become tourist attractions. Tour guides may like to claim that the effect is a mystery or that it is due to magnetic or unusual irregularity of gravity or even that it is a supernatural phenomenon which science can not explain. This is not true of course. Natural irregularities can only be detected with sensitive equipment and cannot account for these places but science can easily explain them as optical illusions. There are several things which enable us to sense which way is up. The balance mechanism in our inner ears is one system we have, but visual clues are also important and can be overriding. If the horizon cannot be seen or is not level then we may be fooled by objects which we expect to be vertical but which aren’t really. False perspective may also play a role. If a line of trees get larger or smaller with distance away, our sense of perspective is thrown off. Objects far away may seem smaller or larger than they really are.

An object rolling uphill is only an illusion because().