The Marathon Man achievement in Halo 3 is a big one that can be unlocked by finding each of the Halo 3 terminal locations in the game.  This is where to find each of the Halo 3 terminal locations.

The back story behind the Halo 3 terminal locations is that the Forerunners have stored information there about their survival and their plan to fire the Halo rings.  The Forerunners are a race that were first mentioned in the original Halo game.  The story goes that they formed an empire more than 100,000 years before the events of the Halo story arc.  Humans contain Forerunner DNA, and therefore can operate the Halo network.  The Halo 3 terminal locations, then, are central to the overall game story.

The game's first level, Ark, has 3 of the Halo 3 terminal locations. The first of these is found as soon as you walk inside. There'll be a door to the left, which is entered to find the first of the terminals.  To find the second terminal look for the structure that enables tanks across the bride of light.  Look behind the bridge controls. It isn't easy to find the last terminal. Following a battle with a scarab there is a large structure, followed by a door and a staircase – look under the staircase.

The level Covenant contains three Halo 3 terminal locations.  Find terminal four by using the elevator up to the tower controls. It takes a leap to an adjacent platform to find the terminal.  Terminal five is accessed via a hornet vehicle, which you use to fly towards Tower two.  Wait for the voice to tell you that Tower two is already clear, that's a reminder that you're going in the right direction. The terminal is located inside tower 2.  The final terminal on this level, terminal six, is in the third tower. Jump to the platform you find there to find this terminal.

There are two additional Halo 3 terminal locations on the final level.  Terminal #8 was a secret until a player found it.  More about the last two Halo 3 terminal locations can be found with a Google search.

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