The final terminal necessary for unlocking the Marathon Man Achievement is hidden on the final mission of the game. Load up the level and start moving forward. As soon as you reach a tunnel area with steel support beams stretching across a bright white sky, drop down the ledge and turn around. You should notice a passageway behind you - and to the left - that leads back in the direction you just came from. Follow the passageway through a door, turn left and the terminal hatch will be right ahead.
Be careful, as there are parts of this secret area that can lead to a fatal fall. And, before continuing or quitting the mission, make sure to nab the gold "Mythic" skull nearby.