Half-naked guy

In the final level "Halo", in Halo 3 there is a secret area for the 7th Marathon Terminal (see Marathon Terminals Page). Aside from the terminal's hallway, there are two rooms: one bright room with a vertical energy beam in the center, and a dark room with a horizontal energy beam and lethal chasm.

It's hard to tell by standing at the edge of the chasm, but there is a small nook on the left side of the room (left, as if one looks out over the chasm). It is not known whether a player can rocket/grenade jump there, but by watching a save film, one can detach the camera and move in there. (edit: you CAN "rocket jump" over to the support piece, and then "crouch jump" to the next one, then you just turn your flashlight on, and zoom in, and you can see him)

In the far corner of the nook is an eerie billboard of a human form facing the wall, apparently with its hands behind its head. Or you could rocket jump up the wall and enter the crack. Turn on your flashlight and you will hit an abyss. Across from that is a floating man wearing green boxers with his hands over his head. If you make it across he can not be killed and has a very strange face. This is also Jason Jones. Ironically, there is a "missing person" poster on the level Floodgate with a picture of Jason Jones on it with its description saying "Khaki shorts and long socks". This can only be attained by "double grenade jumping", or by using a Gravity Hammer in Co-op across the gap.

Jason Jones is also featured amongst a group of grunts charging towards the room where the Prophets are broadcasting in a cameo in the cutscene at the beginning of the Halo 2 Level, Gravemind. He only appears there, half naked as always, in Legendary Difficulty.

Walkthrough
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If this video does not show up on your browser, please follow this written walk-through. Before continuing, please note that this is a co-op walkthrough only.

Begin the level (on any difficulty) and proceed to the room that contains the Terminal. When you get to the Terminal, turn around and you should see an abyss with a beam at the bottom. Be careful, because the next part is easier said than done.

Have one partner stand right at the end of the floor before the abyss. Have the other partner stand on his shoulders. Then, have the bottom partner jump into the abyss. As the bottom partner jumps, the other partner should jump off of his shoulders, and should land on a small piece of floor. Stand to one side, as the partner will respawn right on the small beam that you are standing on. You can then crouch-jump to another beam, and if you turn left and look toward the corner of the nook, turn on your flashlight. You should see the Lonely Soul.

(On a completely unrelated note, you can do another stand-on-the-shoulders jump to get to the next room, which is completely blocked out.