Original by Philip Gibbs, 1996, 1998.
Sometimes you might find or hear of a mysterious place where objects can roll uphill. This is a remarkably common illusion that 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 that 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 might then appear to roll uphill. Sometimes even rivers seem to flow against gravity.
Spots where the illusion is especially powerful often become tourist attractions. Tour guides may claim that the effect is a mystery, that it is due to magnetic or gravitational anomalies, or even that it is a paranormal phenomenon that science cannot explain. This is not true of course. Natural anomalies 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 that 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 that we expect to be vertical but that really are not. False perspective might also play a role. If trees in a line get larger or smaller with distance, our sense of perspective is thrown off. Objects far away may seem smaller or larger than they really are.
People often overestimate the steepness of a slope. If you are standing on a slope of 1° it will seem like a slope of 5°, and if you stand on a slope of 5° it may seem like you are on a slope of 30°. Because of this effect, the anti-gravity illusion can seem stronger than it should be—even though you know the cause.
Even when the true cause is understood, it can be difficult to believe. In some cases the sea horizon is partly visible and it seems incredible that the effect can be an illusion. If you think there is a magnetic anomaly, just use two plumb lines, one made of iron and one of stone. They will hang at different angles if a strong magnetic field is acting horizontally. In fact magnetic anomalies are never that strong, and are never the cause.
But it is not always easy to demonstrate that a slope that appears to go uphill is really going downhill. Plumb lines and spirit levels cannot be relied on if you think there is a gravitational anomaly. If the slope runs parallel to a sea view, it will be possible to compare a plumb line with the horizon; otherwise, the only reliable way of determining the true horizontal is by careful surveying. A good topographical map of the area may show which way the land is really sloping, and the results will confirm the illusion. Gravitational anomalies are always very small. In any case, how would you ever notice a gravitational anomaly?—after all, your sense of balance would be affected equally to how objects would be affected. The anomaly would not be apparent unless there was a clear view of the sea behind the slope, which there never is.
A search on the web turns up a surprising number of examples of this illusion. Most are natural, while others have been constructed in theme parks. Below is an incomplete list of some natural ones. Many thanks to those who have sent more site details for this list. Do check locations are correct before making a long journey and remember that it may be dangerous or illegal to stop or reverse your car on slip roads and bends!
References:
"Skeptical Inquirer", Vol 16, No. 1, 1991; an article about the illusion at Spook Hill.
"Seeing Is Believing? Haunted Shacks, Mystery Spots, and Other Delightful Phenomena" by Chris Banta.