Five years ago, James Therrien was browsing an indie gaming forum when he stumbled across an in-development title that he couldn’t stop thinking about. Called Rain World, the game starred a cute little creature called a slugcat, who is forced to survive a harsh, industrial alien landscape teeming with vicious predators. “I loved the visuals so much, that as soon as I saw it I had this complete audio picture in my head of what it should sound like,” he explains. “It was so bad that I had a nightmare that the game came out and was filled with garbage music.”

Not long after that nightmare, Therrien went ahead and crafted a dozen songs and sent them directly to designer Joar Jakobsson. It was his pitch to do the soundtrack — and it worked. Therrien joined the team as Rain World’s composer, alongside fellow musician Lydia Esrig, and set to work writing music in the chiptunes style the two were known for. (Esrig and Therrien also perform as a duo called Bright Primate.) But as development progressed, the sound wasn’t working out quite right. The bright chiptune bleeps and bloops didn’t exactly fit the bleak alien landscape of the game. What it needed was some junk.

To create Rain World’s distinct soundscape, the composers used a technique known as “junk audio.” Essentially the idea is to use everyday objects — rusted tin cans, old car parts — record the sounds they make, and then mash them together into a song. “You take it, and you process it, and you amplify it, and it becomes something else,” Therrien says of the process. He started out by using field recordings of various objects he found online, compulsively gathering a huge library of sounds. And when the sound wasn’t quite right he went out and recorded his own. “I was using a lot of steel rebar sounds being smacked against each other, but I couldn’t get the right ones,” he explains. “So I’d go out late at night with a recorder and be in the park banging stuff together making these sounds, while trying not to get arrested or raise the ire of my neighbors.”

Once Therrien had the sound he wanted, he’d essentially turn it into an instrument, tweaking it so it could serve as a track’s percussion or bass. He says that the songs in the game were composed primarily using these junk audio instruments. Rain World features three and a half hours of music spanning 160 tracks, and it required thousands of junk sounds to produce it all.

For the most part, the sounds came from mundane objects: brake drums, a car radiator, old paint cans. Sometimes he would record people speaking, and then twist it so that it had a strange alien effect, while still retaining some humanity. “It was to give it it’s own unique language,” Therrien says of the reasoning behind using junk audio. “As if a musical tradition had come out of this world. What sort of musical aesthetic would be created from this environment if given time?”

(In the example below, the thumping bass you hear was originally a five-second-long recording of Therrien saying “I’m literally just recording audio right now.”)

One of the reasons the game has so much music is that it’s not only physically large — spanning a dozen regions and around 1,600 individual areas — but also features dynamic, unscripted events. Much like in No Man’s Sky, the soundtrack of Rain World is designed to respond to what’s actually happening, changing depending on what you’re doing. Therrien describes the system as “threat music.” The idea is that as the slugcat is pursued by dangerous creatures, the music will respond accordingly. If a small enemy appears, you might hear some light drums fade in. As things get more dangerous, the intensity ramps up.

In order to make this work, each track had to be written so that it could be chopped up, with each part standing on its own. Every song in the game is made up of between 8–12 layers, which will fade in or out depending on the context. The idea is to not only make it so the music responds to the action, but also ensure you’re not hearing the same songs repeatedly. “You’re never really seeing a complete piece of the musical picture,” Therrien explains.

One of the reasons junk audio isn’t especially common is because it can be hard to control. Unlike a typical musical instrument, you don’t always know what to expect when you bang a wrench on some old plumbing. (In fact, some of the tracks proved too intense — one had to be toned down after its extreme sounds broke a pair of Beats headphones.) But for Therrien, that’s the point. Rain World takes place in a harsh alien world, and it should have the sounds to match. “Things can get wacky and sound nasty,” he says of the process. “But that’s kind of what I was going for.”

Rain World is scheduled to launch later this year on PC and PS4.