Marathon Infinity is the third and final game in the Marathon Trilogy of science fiction first-person shooter computer games from Bungie Software. The game was released on October 15, 1996 and included more levels than its predecessor Marathon 2: Durandal. These were larger, and formed part of a more intricate plot that spanned both space and time. The underlying engine of the game changed little from the one in Marathon 2, and many levels can be played unmodified in both games. The only significant additions were the Jjaro ship texture set, multiple paths between levels, vacuum-enabled humans carrying fusion weapons (called "Vacuum Bobs" or "VacBobs") and a new weapon. Marathon Infinity, unlike Marathon 2, was originally released only for the Apple Macintosh.
The story in the single-player version of Marathon Infinity, titled “Blood Tides of Lh’owon”, is often considered to be confusing. For example, it begins as if large parts, if not all, of the events in Marathon 2 had not happened. Theories differ on exactly what happens in this game, but the consensus is that the player somehow jumps between alternative realities, seeking to prevent a chaotic entity, the W’rkncacnter, from being released from Lh'owon's dying sun. These jumps are apparently caused either by Jjaro technology or by the W’rkncacnter’s chaotic nature. For example, the player begins the game as Durandal's ally, only to be transported to a reality where Durandal did not capture the player after the events of Marathon. As such, he is controlled by the Pfhor-tortured AI Tycho.
After multiple instances of these "jumps", the player (seemingly the only being who realizes he is being transported between possible realities) activates the ancient Jjaro station, preventing the chaotic entity's release. The ending screen of Infinity leaves the story's resolution open-ended, taking place billions of years after the events of Marathon Infinity.
Despite the player’s being teleported to a Jjaro station by Durandal and left with a grim message in the beginning of Infinity, both Durandal and Earth did survive in the original timeline as can be seen at the end of Marathon 2.
Marathon Infinity was re-released as part of the Marathon Trilogy box set for Macintosh on May 1, 1997.
Bungie released source code and game content from the entire Marathon Trilogy in 2005. An open source project, Aleph One, was launched to allow the three games to be freely played on Mac OS X, Windows, Linux, and other platforms. Other features were added, including online play and support for higher-resolution graphics. The engine was also updated to support Bungie's previous work, such as Pathways into Darkness and the Marathon Zero beta.