ONI Alpha Site
ONI Alpha Site is an Achievement in Halo 3: ODST awarded for completing the Campaign mission ONI Alpha Site on normal or above and unlocks the Alpha Site map for Firefight. It is worth 30 Gamerpoints.[1]
It is represented by an olive circle, with the Marathon symbol imprinted in it, with the remote detonator for an M165 Demolition Charge in front of it.
Sources