Imagine you are walking through a subway station, when all of a sudden you look to your left and there’s a hovering black orb. The sphere is just idling there, seemingly harmless aside from ambient gibberish emanating from its surface. On the scale of things that are creepy, this scenario would probably rank pretty high. And indeed, it does sound like the beginning of a surrealist horror movie.

Not to freak you out: This object is real. But once you understand its mission, the floating black ball is almost disappointingly benign. “A lot of people do interpret it as this sinister moving presence that's following people around,” says Will Yates-Johnson. “But actually it’s meant to be sort of a passive presence.”

Yates-Johnson, along with Francesco Tacchini and Julinka Ebhardt, created the sphere, which they refer to as Space Replay as an experiment in manipulating the sounds found in spaces like stairwells, hallways and elevators. These transitional areas of our buildings are full of unobserved sounds. “In these spaces you have these really brief, powerful moments of sound,” Yates-Johnson explains. "The ball is essentially a tool to capture them."

Inside the 3-foot black nylon balloon is an Arduino hooked up to a microphone and speaker. Every 20 minutes this mini computer records a new track of its surroundings and then replays what it recorded just 20 minutes prior, creating a “delayed human echo.” In other words, it’s essentially a freaky-looking audio recorder. “The fact is it’s a black floating hovering sphere so the visual side does take over,” says Tacchini. “But it remains a sound project.”

In case you’re wondering, no there’s no CGI involved. The balloon is simply filled with party shop “balloonium,” a mixture of half oxygen, half helium. When the balloon reach a state of neutral buoyancy it begins to float unassisted. “If you look closely, it’s not actually stalking anyone,” says Ebhardt. Rather, the orb moves through space, guided only by the natural air current.

The orb is a sonic ghost of people past. The voices and sounds are a reminder of people who just passed through the very same space you are only mere minutes before. In that context, it’s actually sort of a lovely, ephemeral way to connect people who would otherwise just be ships passing in the night. “It’s really about engaging people in a discussion about their environments,” he says. “Especially these places were you pass through temporarily without really thinking about it.”