Rubicon, an add-on for Marathon, was made available for download on St Patrick's Day.

Four years in the making, Rubicon offers over a dozen new characters, 360 new textures, new scenarios, weapons and solo levels for devoted Marathon gamers.

Marathon was originally developed by Bungie, it was an Alien-destroying shoot-em-up for the Mac that developed a major cult following and contributed to the success of the company until its eventual takeover by Microsoft.

The game is available in three different versions, Rubicon Standard, Rubicon AO and Mix 'n Match. The first requires that a copy of Marathon Infinity is installed, the second option is a standalone fully-functioning version of the game. The third version consists of individual files for small bandwidth downloads.