halo 3 marathon suprise *spoilers*
There is a subtle link towards the Marathon game's storyline at the ending of the Halo series. A combat cyborg, Master Chief, is left stranded on a derelict ship in space. In the last unlockable cinematic, the ship is shown entering the atmosphere of a small planet. The ending of the Halo games took place in the year 2553, where Master Chief freezes his core systems to enable him to survive indefinately. Thus it is viable that the main character in Marathon is the Master Chief, revived by the ship's AI upon re-entry. The Halo series is scattered with UESC references, appearing in an awards ceremonys, on ships and other areas on the game. However initally in the series, the ships were branded with UNSC - standing for the United Nations Space Command.