Immediately run forward and up the wall, onto the platform. If you wait too long the floor will open and you will be pulled into space. Break the panel on the right side of the door and go through. Straight ahead as far as you can, and look up. There's a grate there. Climb up the wall, break the grate, and RUN to the end along the wall. At one point some turrets will shoot at you, just run past. At the end there will be two more grates. Break one or both and drop down. Kill the two turrets there. Either door on the side, break the panel. I prefer the left door because it's a little easier to go to the grate. Once the door opens, go inside and turn left (either way) and break the grate. Climb through the grate and up the wall on your left (again, either way). You'll get to an alcove. Run all the way through it to kill everyone, then back to the middle. There are two elevators here. Destroy the power box on the left one and climb into the shaft. Climb up the wall above the door you came through and break the g. Climb through and there are panels on both sides. Either side, break the panel to open the door, and go through. Destroy the panel next to the fan to shut off the generator. Do this to both sides. Climb back into the shaft and up the wall to the next grate. Go through it and there is a panel on the opposite side. Break that panel and go through the door, break the panel to turn off the generator. Turn around and face the ladder. Climb up it from THIS SIDE or you will face trouble with turrets. When you get to the top break the grate nearest you and climb through. Jump down and go through either side passage until you get to the door. Go through the door and turn to the blinking lights. More elevator shafts. Break the power box next to the one on the right. Climb up and break the power box to open the door above it. Be careful because a marine may be behind it, but it's safer on this side than the other. Kill him and go through. There are six doors here, three on either side. The ones on the left will not open The middle right one goes downstairs. We don't need that one. The other two go to the same place. Go through either door, and be careful of the mini-gunner there! Kill him ASAP. Destroy the four power boxes and wait for the fan on the right to stop moving. When it does, jump through it and run through the air duct. At the end jump straight forward right way, don't slow down. There is another turret here. Run straight, take out the two power boxes, turn left. Do that once more, and run straight again. Destroy the turret, and turn right to get the power boxes. Turn around and run back, destroy the last two power boxes. The four fans above you should have stopped. On the ceiling break the grate and climb through to Main 5. Climb up the wall that says "Main 05" and precede through the shaft there. The top cover should be broken, if not you're in the wrong shaft. Climb up, go up the slope, and drop down the hole in the floor. In these rooms just keep breaking the glass and moving the same direction you were facing. There are nine of these rooms. In the last one climb up the wall and through the hole in the ceiling again. Go down the slope and destroy the two power boxes once more. Wait for the fan to stop, and run through it. Destroy the grate on the left and run through the air duct. Now, the next room can be challenging. You're near the end. When you enter, an alarm will sound and the walls will begin to open. There may be mini-gunners here, so watch out. Climb up one of the walls and kill anyone there, and then destroy the light/computer there. Go to the other side and do the same. You'll see a message that says "Join us." and begin to hear other aliens. On the far side of the room from the vent you'll see the "Danger" bars move away. Run through to join the rest of the hive and end the level