Gravity hills go by a few different names: mystery hills, magnetic hills, spook hills, magic hills etc. And despite fun theories of supernatural forces at work, it’s an illusion caused by the same environmental factors. The most important one is that the horizon must be partially or fully obscured. As a result, trees on either side of the road appear to bend, and our typically reliable brains get confused by the gradient of the slope. The shape and size of surrounding hills can have some influence too. 

We like to think that our perception can be relied upon, that something must be so because we saw it with our own eyes. In truth, the perceived reality is the result of our brains filtering and making sense of non-stop sensory inputs. Most of the time, our brain constructs a reality that accurately matches what is actually going on. To do this efficiently, we will often unconsciously fudge our perceptions to match an expectation, filling in the gaps from past experience, and predicting what we should see or what should happen, just because it usually does. If and when it doesn’t, the result are well-studied biases, mental blind spots, and illusions that magicians use to their advantage. Examples include our brains predicting motion that doesn’t exist with the ‘flash-lag’ illusion. Ponzo optical illusions mess with our sense of distance; the Hering Illusion appears to bend straight lines, while the Bistable Optical Illusion allows us to see both clockwise and counter-clockwise movement at the same time. You can see hundreds of additional examples here, but you won’t be able to drive a car in them.

Watch how the illusion works