Dead Island’s success apparently revolved around the co-op experience.

The title, which shipped 4 million units, did so according to the cooperative gameplay, according to Guido Eickmeyer, development director and executive producer of games at Deep Silver.

“This is why we got all the good reviews from users. This is why we got high user ratings, because it is the most exciting co-op experience that is on the market,” Eickmeyer said.

The game, which cost less than $25 million to make, has been played for over 56,908,547 hours.

A sequel, Dead Island: Riptide was revealed at Gamescom.