Alpha Labs - Sector 2: Union Aerospace Science Division is the sixth level of Doom 3. Trites make their first appearance soon after the first web sighting in the exit room. There is a glimpse of Bravo Team through a window. This is where the dark labyrinth with the lantern guy is located.
At your arrival, you are "greeted" by a lot of Medkits and a levitating corpse you won't be able to follow. After that, the Marine will crawl a lot in this level. The first time is access vent from the water closets (there is no "mirror scene" ini this one) ; on your way, you'll be able to observe, through a grid, a zombie eating a fat zombie's rotten guts. Once you drop from this vent, there is no return, so you can collect all plentiful ammo and medkits there is at the beginning of the level. Just before reaching what is presumed the first part of the Molecular Fuel Storage Compactor, Marine will see the Bravo Team running through a window, albeit unable to communicate with them. Sergeant Kelly will then assignate the next objective : connect with Bravo Team in Sector 3 of Alpha Labs. Marine has then to make his way in the dark labyrinth of cooler section, where a scientist guide him with a lantern. The Chaingun (among other bonuses, such as Backpack) can be acquired here. Then, there will be another crawling section, which is another point of no-return, before the final trial of this level (and perhaps the hardest so far, so it might be a good idea to save here) : the Trite Room. The best way to survive seems to stay at under the ladder case (between the computer and the wall) and use the Chaingun ; if needed to reload, switch instead to Machine Gun. Once Trites are eliminated, the ladder appears. There are som more Trites up there ; just open the door to the left, run into and either gun the Trites from here or directly exit the level.