The Aleph One project has a limited license to distribute these files. Unfortunately, Bungie™ has not released Marathon game content to the public under a formal, unambiguous content license. To our knowledge, Bungie has not blocked any noncommercial distribution of these assets, but the Marathon series is not considered abandonware and Bungie retains the right to control its distribution and use.
Content in the "Plugins" directory comes from a variety of sources. Please see documentation within each plugin.
In 2000, Bungie released the Marathon 2 source code under the GPL 2 license, which led to the Aleph One project. The game content was not part of this release; the games were still commercially available at the time.
In 2005, Bungie made the Marathon game content freely available, at trilogyrelease.bungie.org. No content license was posted. The Frequently Asked Questions page includes this statement:
Wow... can I do whatever I want with this stuff?
NO. Bungie still holds the copyrights to these files. They're allowing them to be distributed for free (mostly because you can't buy them any more) - but they're still Bungie's intellectual property. You can't, for example, sell them.
In 2011, Bungie released the Marathon Infinity source code under the GPL 3 license. The source code archive also included a CC-BY-NC-SA 3.0 license, but its scope is unclear. It may only cover the design documents and other non-code files present alongside the source, and not the game data (which was not part of the archive).
In late 2011, Aleph One began distributing Marathon game content bundled with Aleph One binaries, in the spirit of the Trilogy Release page's "free distribution" aim. Bungie announced these bundled downloads on their company site, so Bungie was aware of and tacitly approved Aleph One's redistribution of the game content.
In 2021, Bungie granted the Aleph One project a limited license to distribute Marathon game content.