Farthest Outpost
From Halopedia, the Halo wiki
200px Farthest Outpost is a song included in the Halo 3: Original Soundtrack. It plays during the beginning of the Halo 3 level The Ark.
The beginning of the song is based mainly on choir and strings. At around 0:40, drums become the dominant instrument. Later on, the melody is taken over by strings again. After that, a section from the songs, Out of Shadow and the Halo 3 version of Perilous Journey will play.
The name of the song is most likely in reference to the Ark, which could be considered the "farthest outpost" of the Forerunners.
The song runs for 5:14.