Gravity, we're told, is just a theory. Or at least, in science we use theories of gravity to explain why objects tend to fall towards the Earth. Gravity is the theory; that objects tend to fall towards the Earth is the fact.

But how does science explain weird places where gravity doesn't seem to apply? For instance, scattered around the world are a number of strange, physics-defying "mystery spots," places where objects appear to roll uphill rather than down, where cyclists struggle to peddle down rather than up, reports Science Alert.

These places are known as "gravity hills," and many of them, such as California's Confusion Hill, have been transformed into uncanny roadside tourist attractions. Perhaps understandably, these natural phenomena have also come with their fair share of wacky explanations, from witchcraft to mysterious spacetime vortexes to conspiracy theories about giant magnets buried in the hillsides.

A much simpler answer

The real explanation, it turns out, is pretty simple, but you might still have a difficult time believing it. Take, for instance, this anti-intuitive gravity hill found in Aryshire, Scotland, which was investigated by researchers and covered by the Science Channel:

Cars on this road appear to roll uphill, a spooky mirage that has long confused anyone driving along it. But when a road surveyor was asked to take precise measurements, there was nothing spooky about it at all. The end of the road that seemed like it was uphill was actually downhill. So despite appearances, gravity was working exactly how it should.

In other words, gravity hills are really just optical illusions. Your brain is being tricked into believing that up is down and down is up, and it comes down to a simple matter of relative perspective.

"We're standing within a tilted landmass," explained U.K. psychologist Rob Macintosh in the Science Channel video above. "The whole landscape tilts this way, and the road tilts in the same direction, but by a smaller amount, so the relative slope appears to go the [opposite] way."

The phenomenon can be illustrated with this simple model, where lines from the landscape fool our brains into drawing an incorrect horizon:

gravity hill How gravity hills trick us into believing that up is down and down is up. (Photo: Science Channel/YouTube)

Gravity hills like this one almost always appear in places where the actual horizon is obscured, which forces our brains to make one up based upon other observational cues. Some of these illusions are more convincing than others, though. Take for instance this gravity hill in Pennsylvania, where the road appears to intersect with another road at a lower elevation:

It's a mind-bender, to be sure. But when the YouTuber tested the road with a carpenter's level, the road proved to be tilted in the direction predicted by the rolling objects. So the mystery is in our minds, not in the natural world. Like so many other things, it all comes down to a matter of perspective.

It goes to show that perhaps seeing shouldn't always equate to believing after all.