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 onto the Earth's surface. At some point, the Chief receives reinforcements from Pelicans. He defends them as they attempt to land, but not well enough as one of the Pelicans gets shot down. About a third into his mission, he runs into Marines and confronts the Arbiter. This is where it is believed that the Chief and the Arbiter first join forces.

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.