Why Am I Here?
From Halopedia, the Halo wiki
The message can be found behind the several small rocks near the top of the map, above the Sniper rifle. The achievement is worth 5 Gamerscore.
The name of this achievement, as well as of the Easter egg itself, is a reference to a running gag in Red vs Blue. In this joke, Simmons asks the question "Why are we here?" and Grif interprets it as a deep philosophical question and responds as such. Simmons then says that he simply meant why were they living in an empty, meaningless canyon.