Virtual reality rhythm game Beat Saber may just be the first VR title to sell more than 1 million copies: Beat Saber surpassed the one million unit milestone in February, according to Beat Games co-founder and CEO Jaroslav Beck, who credited word-of-mouth for most of the game’s success in a recent interview with Variety.

“We did zero PR whatsoever,” Beck said. “We didn’t expect that it would blow up that crazy.”

Beat Games also announced the release of its first song pack Thursday. Dubbed “Monstercat Music Pack,” the paid add-on allows Beat Saber players to add ten new tracks to the game. As the name suggests, all of the tracks are from the Canadian EDM label Monstercat.

Beat Saber has become a hit with VR users ever since its release in May of last year. The game can best be described as a mixture of “Fruit Ninja” and “Guitar Hero,” requiring users to slice squares to the rhythm of the music.

“It is super intuitive,” said Beck when asked about the secret to Beat Saber’s success. He also credited the technical chops of his team for Beat Saber’s popularity, saying that the game didn’t crash, and didn’t lead to motion sickness. “The code is super polished,” Beck said.

Thus far, “Beat Saber” has mostly focused on electronic music, but Beck said that his team was looking to expand the game’s music choices over time to other genres as well. “We are definitely experimenting with everything.”

Some of the music will be released in future music packs, and Beck said that he had been getting a lot of inbound requests from music labels. However, he also committed to regularly add free music to the game that Beat Games would publish on its own. “We are almost becoming a label by ourselves,” he said.