From Halopedia, the Halo wiki
Inundation is an achievement in Halo: The Master Chief Collection. It is awarded for activating the second Halo 3 Terminal on The Ark. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty. The player must scroll through and access both parts of the terminal to unlock the achievement.
The terminal is found immediately after Rally Point Alpha. Inside the structure which the player must enter to let the tanks across the energy bridge, the terminal is directly behind the controls for the bridge. The bridge must be activated before the terminal can be accessed.