TerminalsThe Terminals in Halo 3 are Forrunner computers that are shaped like giant eye balls and can be found throughout the Forrunner based levels of the game. When a player accesses a terminal, a screen appears with part of a story about two Forrunners during the time just before the Halo rings were fired and the Forrunners were fighting to stop the Flood. A bit of this story is found in each of the 7 terminals throughout the game.
The terminals can be found and accessed on any difficulty in the game. To unlock the Marathon Man achievement, you have to wait or go through each terminal until AFTER the alarm goes off, the screen turns red and you get redirected within the terminal (the redirection occurs automatically after a certain amount of time). You do not have to access the terminals in any order and you don't have to access them all in the same game session.