Before there was Halo and Destiny, there was Marathon — and now Steam users can check out the classic sci-fi shooter for themselves. Bungie just announced that the first game in the original trilogy has surprise-dropped on Steam, and it’s available on both Mac and PC. Even better, you can play it for free.

The game is actually credited to Aleph One Developers, which is described as an “open source continuation of Bungie’s Marathon 2 game engine.” In fact, the group has already made the entire trilogy available to download right here with support for Mac, PC, and Linux. But the Steam launch opens up the games to a big new audience of gamers, and Bungie says the plan is to bring both sequels — Marathon 2: Durandal and Marathon Infinity — to Valve’s platform as well.