People have active imaginations.

Not only that, our brains are hard-wired to try to make sense of the world around us – but that doesn’t mean that sometimes they get the explanation wrong.

For years, people have reported seeing “dark watchers,” or misty, dark shadows that seem to follow them when they’re walking or running outside.

Here’s why you might be able to see them, too, especially on a foggy, misty day.

“Dark watchers” are an optical illusion that’s officially known as a “Brocken spectre.” Named by German scientist Johann Silberschlag, who spotted one in the Harz mountains, they have caught the imaginations of both everyday people and noted authors like Charles Dickens and Lewis Carroll.

Despite the fun imaginations of men like this, there’s nothing sinister about the figures, who are particularly easy to see when it’s foggy.

That’s because they appear when a person creates a shadow that’s cast onto a cloud or mist, which results in a larger-than-life silhouette that appears to be far away. The magnification appears to be touching the farthest parts of the landscape, and can sometimes appear to move even if the person stays standing still.

You can imagine that a giant, shimmering shadow that moves independently in the mist would really have freaked some folks out before we had the science to explain what caused it.

Now that you know, though, you can feel safe searching for one of your own.

If you think that’s impressive, check out this story about a “goldmine” of lithium that was found in the U.S. that could completely change the EV battery game.