Call of Duty: Black Ops fans have uncovered a new cut Zombies mode from the game, over a decade and a half after its original release.

YouTuber JTAG uncovered a tower defense Zombies mode in the recent PS4 port of Black Ops 1 and uploaded gameplay of it. It’s clearly unfinished, so the mode is a bit crude, but players would’ve been able to place down turrets to stop hordes of the undead from advancing through a maze-like map. It seems as though the player would’ve been able to look down at the action from a bird’s-eye view, not unlike Bloons Tower Defense, but they also would’ve been able to get on the ground and shoot enemies just like regular Call of Duty.

It’s possible this was scrapped in favor of Dead Ops Arcade, a hidden Zombies mode in the original Black Ops game. The mode was a twin-stick arcade shooter that players could access by using a secret computer terminal in the main menu of Black Ops. The mode became a cult-classic and has returned in various Black Ops titles over the year.

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Treyarch has experimented with the Zombies formula a lot over the years, and it’s not exactly a surprise to see something like this turn up. The original Zombies mode was made by a small team of passionate developers who were developing the project in secret. It wasn’t officially sanctioned by Activision and used recycled assets from Call of Duty: World at War. There was zero guarantee it would even make it into the final game, but once the team presented it to executives, they realized it was special and worth tacking on. And now, it’s one of the most beloved pieces of the Call of Duty franchise.

It wouldn’t be surprising if this tower defense mode was one of many experiments done to try and find more weird ways to expand the mode. It seems unlikely it will ever return, but it’s cool to have a peek behind the curtain, especially when it’s remained a secret for such a long time and may have never been discovered if not for the recent ports.

Cade Onder is a freelancer for IGN’s news team. He covers all things entertainment, including gaming, film, and more. You can find him on Twitter @Cade_Onder.