Here is an interesting comparison of Cortana over the course of the 4 halo games:
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Marathon Man (All Halo 3 Terminals)
The Marathon Man is a 40 gamer score achievement in Halo 3 which you can unlock by finding and accessing all hidden terminals in the campaign. Terminals were a series of computers stations and teleporters found in the Marathon trilogy, a series which has contributed Easter Eggs to all of Bungie later games.
The first terminal is found the first time you walk inside (in a small hallway). The first time you go inside, you will see a door almost immediately on the left. Simply enter this area of the level to find the first of the seven terminals.
When you go inside the structure to let the tanks across the bridge of light, there will be a Terminal almost directly behind the controls for the bridge. It?s an easy find!
You?ll miss this terminal if you are not careful. After defeating the scarab, you will head into a large structure. There will be a room with several sleeping grunts. Nearly directly following this room is another room in which you can take two different directions. There is a door, and a staircase (both eventually lead to the next room). Underneath the staircase is an additional door that leads to the 3rd terminal.
After you have shut down the first shield generator, go down in the lift as usual but when you get to the bottom don't jump off. On one side there will be a room you can jump over to from the lift. The terminal is in there.
After you have got your hornet do not fly to you objective yet, along the way there will be a forerunner installation jutting out of the cliff to your right. Land there and at the back off the building in a room you will find the terminal, you can find and claim the Thunderstorm skull if you haven't got it yet it's located near by.
When you have disabled the last shield generator come back down the lift and to one side there will be a room you can jump over to, there terminal is inside.
Near the beginning of the level continue as usual but when you come to the part where the path slopes down to the base of the control room take a right turn into a small passage. Cortana will say something like "Chief where are you going?", ignore her and keep going. At the end of the passage way Cortana will say "wait go back". Take a left and the terminal will be there.
Happy terminal hunting. And remember to stay in the terminal until it kicks you out or it will not save and you will end up not getting the achievement.