Secret Marine on The Covenant

In Halo 3, on the level The Covenant, when you start at Rally Point Bravo, Grab a Mongoose and drive straight ahead at full speed. When you get to the part where a Marine says, "Citadel in sight". Get off of your Mongoose and grab a Ghost as soon as possible. Drive the Ghost ahead as normal. When you get near a tunnel, turn right and climb straight up the mountain. Occasionally, when you climb up the mountain, there may be two Marines. Also, the Marine will only appear when you play on local and with only one player.

You can also do this in a Prowler - steal one from the Brutes in the area before the tunnel. There are usually two Prowlers, but once you get to the ridge on the top, you will need to pick your path carefully; some parts of the final ascent are too steep for a Prowler. Also, the Prowler is very slow so by the time you get to the top, the Marine may be gone because you have a time limit for reaching the marine. However, once you reach the marine, he will always be there even if you go back down the mountain. Depending on how fast you reach the marine, there may be 2 marines on top of the mountain.

Video
This is a video showing you how to get to the secret marine: 300px

Marine Details

 * The Marine is similar to the one that appears on 343 Guilty Spark in Halo: Combat Evolved. However, the Marine will usually change faces every time you go up the mountain. The varying faces of the Marine may be the faces of developers from Bungie.