The solemn and disturbing side-scrolling adventure Inside is one of the year’s best games, and also one of the creepiest. And it turns out that eerie nature extends to the game’s development. In an in-depth article on Gamasutra, Martin Stig Andersen, Inside’s composer and sound designer, revealed that he recorded the game’s haunting soundtrack with the help of a human skull.

Andersen says that the decision was made because he was interested in the way your voice sounds different inside your own head. He wondered how that kind of audio filter would translate to Inside’s music. “That was the curious thought that led me to acquire a human skull and experiment with it,” he writes. Andersen and his co-composer wrote songs aiming for a 1980s synth vibe, and then piped them through the skull using a contact microphone. While he says that the sounds initially came out sounding poor, he was able to clean them up afterwards.

“I did some post-processing as well, of course, but even without it you can hear this kind of timbre, which inspire a very nuanced association with a synth soundtrack,” Andersen says. “During the development we acquired an intuition for what kind of soundscapes resonated well within the skull.”

While he notes that the bizarre process gave Inside a unique sound, Andersen says he doesn’t expect to use a skull again for future soundtrack works. One reason might be its fragility. “Eventually all the teeth fell out of the skull because of the vibrations,” he writes, “but while they were still there they created this small vibrating sound that I think was unsettling but also strangely familiar to people.”