If you’ve ever wanted to see a ball roll uphill, we think you’ll be amazed by the natural optical illusion known as “gravity hills.” These strange land formations can be found all over the world — including California, Canada, and Scotland — and they all invite the eyes to follow a spooky yet beautiful path.
Despite its common nickname, we think these spots should be called “antigravity hills,” as objects here seemingly drift uphill on their own or struggle to move downhill. Obviously, that’s the exact opposite from what we’ve come to expect out of hills, so it might shock, confuse, or even scare you if you saw it in real life. For example, watch what happens when you try to roll a ball one way on a gravity hill in Pittsburgh in the popular YouTube video below.
Or take a gander at this gravity hill in Canada and see what happens when a driver puts a car in neutral while on the road.
As Science Alert reports, the ball isn’t actually moving uphill in the first video, and the car in the second video isn’t actually drifting up the hill, either. This is just a natural optical illusion that’s so impressive, you need special equipment to get to the bottom of it (unlike the actual hill itself!). Fortunately, experts have solved the mystery with the help of GPS markers and surveying tools. The trick is measuring the difference between the “top” and “bottom” of the hill, which in reality are actually reversed.
“The embankment is sloped in a way that gives you the effect that you are going uphill,” materials physicist Brock Weiss from Pennsylvania State University said in an interview with Discoveries and Breakthroughs in Science. “You are, indeed, going downhill, even though your brain gives you the impression that you’re going uphill.”
Something else that plays a role in making gravity hills such a great optical illusion is the horizon line. As you may have noticed in the videos, the horizon is either curved or totally out of view. As researchers of a study published in Psychological Science revealed, “We conclude that antigravity-hill effects follow from a misperception of the eye level relative to gravity, caused by the presence of either contextual inclines or a false horizon line.”
Phew! We’re glad we’re not imagining things.