Sierra 117

Sierra 117 is the first campaign level of Halo 3.

Upon completing it on the Normal, Heroic, or Legendary difficulties, the player will earn the "Landfall" achievement and 20 Gamerpoints.

Setting
It takes place in an African jungle, as confirmed via the hexadecimal code of the Halo 3 Beta and physically confirmed in the Halo 3 E3 2007 Trailer and subsequent Bungie Podcasts. The jungle is home to a Covenant Loyalist encampment, and hosts lush verdant forestry, rivers, waterfalls, and tunnels.

Plot
According to the Halo 3 E3 2007 Trailer, somehow the Chief arrives in this jungle at the beginning of the game and joins forces with the Arbiter and a few Marines. They progress through the area, securing landing zones for Pelican Dropships and defending them against enemy Banshees. If one makes inferences based on the Halo 3 Zune Concept Art and the beta code's level names, the level will culminate at an arrival at the Halo 3 UNSC Base.

Trivia

 * Note that the level is called Sierra 117; Sierra is the word used in the NATO phonetic alphabet to represent the letter S, so it is S 117 which corresponds to SPARTAN-117. This has been confirmed in the latest bungie podcast.