To access the terminals in Halo 3, a player must approach the terminal, then press and hold the right bumper (RB) button on the Xbox controller. Players need to scroll through all the messages and wait until the green arrow or terminal screen changes color to red, otherwise accessing the terminal does not count toward in-game achievements.
There are seven main terminals located throughout the Forerunner levels of Halo 3. When accessed, each terminal provides part of a back story to the overall game that revolves around the tale of two Forerunners living in a time prior to the first firing of the Halo rings, during the war with the Flood. Accessing all seven terminals unlocks the Marathon Man achievement and earns the player 40 Gamerpoints.
Three terminals are located within Mission 6 and three more within Mission 7, while the last terminal is located within Mission 9. Terminals can be accessed in any order and on any difficulty setting while in Campaign mode. Failure to access the terminals does not prevent players from successfully completing individual levels within the game or the campaign as a whole. According to Halo Nation, an additional eighth terminal is located on the Cortana level. Accessing this terminal is not required in order to unlock the Marathon Man achievement.