What you see is what you get. Right?

Well, maybe not.

A lot of things happen inside of us when we see something. Our brains are constantly trying to make sense of the world around us. It uses our expectations and memories to help in the process. Often that is a great help, both in processing time and accurately determining what is happening. However, occasionally our brains get it wrong.

Is that a big deal? Ask someone accused of a crime. A crime that relies heavily on eyewitness testimony. Researchers know eyewitnesses can be shockingly inaccurate. However, eyewitness testimony remains a heavily influential part of a case. Yikes!

When we observe an event, our brain immediately tries to make sense of it. Right or wrong, the memory is created and stored. When we try to recall an incident, we don’t go back to the image. We just pull up the memory.

So be wary of the tall tales you may hear about first hand experiences. You’ve heard the line about there being 2 sides to every story. Both sides vehemently defend their stance, however no one was seeing what truly transpired.

We know our perceptions are flawed. Even when we know things aren’t as they appear, we still get fooled. Enter the magician or as many refer to themselves, illusionists. They make a living taking advantage of the way our brains work. Armed with a sense of humor about ourselves, it is great fun trying to figure out how they manipulate our understanding.

We see evidence of playing with the senses throughout history. The ancient Greeks manipulated perspective in their architecture for a more pleasing aesthetic. A few of the great Dutch masters used tools to paint “realistic” images in a way that the human eye can’t even discern.

(See related: Vermeer paintings: More than meets the eye?)

So, what is real? This is where science can help us out.

This article takes a look at a baffling phenomenon caused by an illusion. Gravity hills. Our perception is fooled to literally not know which way is up. Intrigued? Read on.


What is a gravity hill?

Are you familiar with the term gravity hill? Also known as magnetic hill, antigravity hill, or mystery hill.

I was unfamiliar with this until I saw a video of a gravity hill recently. Fascinating.

A gravity hill is a road where objects on the hill “defy” the laws of gravity. Thus, the name. Objects such as liquids, balls, bikes, and cars in a neutral gear will effortlessly flow or roll uphill. Likewise, effort must be exerted to move downhill. It is as if gravity was reversed.

These gravity hills or “mystery spots” often have fantastical stories associated with them to explain the puzzling behavior. Creepy stories that involve mischievous actions of ghosts. A mysterious cosmic vortex causing the laws of physics to be bent. It all depends on the local lore.

Well, the laws of physics aren’t really being broken. And if it is ghosts, they are indiscriminating and remarkably steadfast.

Of course, it is just an optical illusion being played on our senses. Our perception is the thing going haywire.


What causes this phenomena?

There can be several things that aid in the optical illusion of a gravity hill. Each instance may have one or many of these conditions:

  1. Obstructed horizon. If the observer is not able to see an uninterrupted view of the horizon, there is no reference point for a true level. In that instance, you need to use the surroundings to make your best guess. The whole landscape may be tilted and jagged. It is nature after all!
  2. Road is cut into existing landscape. The creation of the road may have required grading due to inclines/declines or rough terrain. This causes the road to contrast with the surroundings and could set up the illusion.
  3. Background scenery is curved or tilted. If your reference “horizon” is anything but level, your mind determines level. Your inner ears and eyes need to negotiate what makes sense. If you don’t, you may feel dizzy or nauseated. That is why those suffering from seasickness are told to look at the horizon.
  4. Natural surroundings. Trees will grow in the direction of sunlight so they aren’t always a perfectly perpendicular reference. Buildings, however, are built to be level. Once again, without a proper level reference, you are left to your own devices to figure out your surroundings.

When several of these factors are combined, it can create the “perfect storm” of optical illusions. You will use your surroundings to make sense of the world around you and that will leave you with an optical bias. An incorrect optical bias.

There is an interesting application of this knowledge. When roads have a large dip, or change from downhill to uphill, it tends to cause traffic jams. To prevent traffic congestions, engineers may design the road to make it appear that the uphill is not uphill. They can use tools like grading and roadside surroundings to change the visual illusion.

Watch this clip to see the experience of a gravity hill and get a visual explanation.



Debunk the myths

Are you a skeptic of the ghost stories or other mysterious explanations surrounding gravity hills? If so, there are a couple ways to disprove the hype. They range in easy to more involved.

  1. Use a level. The bigger the better because road inconsistencies may show a small localized area to act differently than a hill that is dozens of yards long.
  2. Use GPS (global positioning system). There are stand alone units as well as applications you can use on your phone. They may not be remarkably accurate, but should give you enough data for what you want (elevation at the perceived “top” of the hill and elevation at the perceived “bottom” of the hill).
  3. A topographic map. This can show you the relative elevations of the local area.
  4. Survey the hill. This is obviously more involved. It requires knowledge and tools to do it correctly. If you have access to both, that’s great.


Places with gravity hills

There are many instances of gravity hills around the world. Here is a map of just a few in the USA and Canada. For a more comprehensive list, see the Wikipedia page for gravity hills.

Gravity hills in the USA and Canada

courtesy of Google Maps


Related phenomena

  1. Ames illusion. A specially built room that creates the illusion that objects in the room have a dramatically distorted size. To read more, visit this Ames Room site by the Illusion Works.
  2. False flat. Bikers may be familiar with this term. It is when a stretch of road appears to be flat, but in reality is a slight incline. You can imagine that this would be confusing and require a lot more energy than a tired biker anticipates.
  3. Gravity anomalies. Local variations in gravity do occur because of topographic and subsurface density variations. Regardless of what is causing the variation of gravity, it is quite small at less than 1% of gravity. Humans wouldn’t notice these variations where they would notice the several g forces encountered on a roller coaster.


This illusion stuff is really fun. You know better, but your mind is trying to make sense of it the best way it can.

Just for fun, here is a video showing 10 types of optical illusions.



Do you have a favorite type? Do any of them make you slightly queasy?

Have you ever visited a gravity hill or mystery spot where your senses play havoc on you? Where was it and what was your experience?

Let me know in the comments.



10 Mind Blowing Optical Illusions from YouTube

Ames Rooms from Illusion Works

An examination of the Bedford County gravity hill from M.A. Frizzell

From psychology to real life: Envisaging the practical application of visual illusions from Ritsumeikan University

Glossary of cycling terms from Century cycles

Gravity Hills from M.A. Frizzell

How do “mystery spots” and “gravity hills” work? from io9

I know a place where things seem to roll uphill.  How does it work? from Philip Gibbs

Lecture 4: Gravity anomalies and isostasy from School of ocean and Earth science and technology, University of Hawai‘i at Manoa

The eyewitness expert from Eyewitnessexpert.com

The road where cars roll uphill from the Science Channel

Why Science Tells Us Not to Rely on Eyewitness Accounts from Scientific American

Wikipedia contributors. “Ames room.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 27 Sep. 2015. Web. 9 Oct. 2015.

Wikipedia contributors. “Forced perspective.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 5 Aug. 2015. Web. 9 Oct. 2015.