There’s a place in Lake Wales, Florida, that draws curious tourists and seekers of mystery from all over the world. It’s called Spook Hill, and the name describes the experience pretty well. Stop your car at a painted white line in the center of the hill, put it in neutral, and let it roll—upwards!

Spook Hill is just one of hundreds of such hills scattered throughout the world. They’re called “gravity hills” or “magnetic hills,” and there are as many stories and explanations for the strange phenomenon as there are locations to watch it work.

So how is this possible? How can gravity be reversed, such that balls roll uphill instead of down?

Well, that depends on who you ask.

According to the Spook Hill legend, the whole thing started hundreds of years back, when giant alligator threatened the existence of a Native American city near the lake. A hero chief from the town battled the gator to a standstill, and both lost their lives in the final contest. The people buried the hero on the north slope of the hill.

“Later, pioneer haulers coming from the old army trail atop the ridge above found their horses laboring here…at the foot of the ridge,” a sign at Spook Hill explains. “Is it the gator seeking revenge, or the chief protecting his land?”

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Wikipedia

The correct answer, of course, is neither. Gravity hills don’t have anything to do with the supernatural, or with gravity for that matter. They’re just bizarre optical illusions.

You need a few ingredients to make a proper gravity hill.

First, you take an obscured horizon. Straight horizon lines will ruin the illusion. Next, you either need a road that travels over two hills, one right after the other, or else a surrounding made entirely of downhill slopes. Trees that grow at an angle to better capture the light help out a lot, too.

With all these ingredients in place, viewers will perceive the road to have a slight uphill slope, when in fact the opposite is true. This illusion consistently works, even when you know it exists. A team of researchers from two Italian universities found that they could recreate the illusion in a lab setting.

In fact, their research shows that the human mind has a strong tendency to view hills as being tilted upwards rather than downwards.

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iStock

The researchers created a “landscape” out of boards. The central board was tilted downwards, while two boards at the outer edges were tilted even further downwards. Everyone who looked directly at the scene saw the central board as tilted upwards.

Then they tried the experiment again and again, each time with a different arrangement of boards. In every case, people saw the board as either tilted upwards or totally level. The takeaway is that it’s hard to get the mind to see a downhill slope.

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Prezi

If you’ve ever visited a tilted room, such as The Mystery Spot in Santa Cruz, California, you’ve experienced the same sort of visual illusion. To create this illusion, builders tilt a room by 20 degrees or more. People who stand in this room look oddly tilted, even beyond what’s possible given the typical rules of gravity. Really, it’s just the odd tilt of the background creating this illusion.

So, as it turns out, the “spook” in Spook Hill is human perception.

It’s sort of like a related phenomenon called “looming,” which caused a lot of confusion at the 2013 Scottish Open golf tournament.

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Golfweek

Video footage of the event showed a crowd sitting and watching the game, while a large industrial ship seemed to float overhead. Lots of people tweeted about the “flying ship,” but eventually the experts weighed in with the actual cause of the illusion.

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The Maritime Executive

Looming occurs where pockets of warmth sit atop cold water. The temperature difference gives the air different densities. As light passes through the varied densities of air bends the light, making them look much higher in the field of view than they actually are.

When that happens, you get floating ships.

At the time, Buzzfeed reached out to Sky Sports, the network that broadcasted the Scottish Open. They asked about the phenomenon of the “flying ship,” and got an answer that matched the question pretty well.

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The Maritime Executive

“A Sky Sports spokesperson denied that it was a flying ship, and added, ‘This is a ridiculous story,'” Buzzfeed reporter Tom Phillips wrote.

If you’re interested in optical illusions, Wikipedia offers a nice list here. They cover all the types of common optical illusions, including cognitive illusions, in which the mind interprets visual signals in a way that conflicts with objective reality. Other types include physiological illusions, which actually hijack the neural pathways of the brain. An example of this type of illusion would be the “afterimage,” as when you look at a bright light and still see a spot when you look away.

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iStock

Gravity hills are a typical example of your standard cognitive optical illusion. If anyone tries to tell you otherwise, they’re selling myth, not fact. There are no ghost children pushing your car off the railroad track. There is no spirit alligator and no long-dead hero spirit—just people and their weird, weird brains.

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