When you enter the level, switch to electrical view and draw your pistol. Pull the switch on the right to open the door in front of you. Go through, but not too close to the fan - you'll get sucked up and die. Head left, down the stairs and up the steps ahead. Take the lift on your left, then follow the path round until you reach the top, where there's a switch. Pull it, then fall down and make your way back to where the fan is. This time go straight across and through the door, then wait. Some aliens will break down the wall at the end, maybe setting themselves on fire. Kill them then continue up the stairs at the end. When you reach the top, shoot the centre of your pipe with your pistol. Jump into the gap and walk along to the end, falling down the hole. Hit the switch there to deactivate the fan, then the switch next to the door to unlock it. Head left and through the door to return to the area with the fan, which has now stopped. Go through the fan and there's a door behind it which opens. Continue down this passage until you reach a large room. There are two doors here - you need to take the left one. Follow the corridor round, up a few flights of steps, until you arrive at a towering great room. In the far right corner from you is a switch. Hit it, then fall down the hole next to it. Go through the door in the corner (not the taller one - that takes you back to the fan) and wait. A couple of aliens are positioned behind some barrels, and they usually set themselves on fire. When they're dead, continue down the corridor and go up the stairs. When you reach the top follow the path round until there's a door on your left - take it. Go up the steps and through the door and you're in that big towering room again. Go across to the other side and flick the switch, then return. Exit through the door and head left, following the passage round until you reach the mess hall. Collect the field charge, then head out and right through the door. Go across the room and up the spiral staircase, then through the door at the top. Follow the corridor round to the towering room, then go right, along the ledge and flick the switch. Return to the room with the spiral staircase, then go right, along the ledge to enter the door on the right. Walk forward a bit, and an alien will run straight over you and out the door for some strange reason. Ignore it, and ride the lift up. Go left through the open doorway and then through the next door. Now go right, along the ledge and hit the switch. Follow the ledge back around and enter the lift. When it reaches the bottom, go across the room and walk up the steps in the far right corner to enter the pipe. At the end, go right, then take the first right. Hit the switch inside, then exit the room and head right yet again. At the end of the pipe go through the huge doors, then take the lift up at the end. Select your disc. A Praetorian will emerge from behind a crate on the left at the top, so pulverise him. Retrieve your disc and switch to thermal view. Cloak and select your Speargun, then go up the steps and look right. There should be a marine you can see - try edging forward bit by bit. When he's dead, go down the stairs and follow the ledge around until you see a lift in the far corner. Ride it down and kill the marine at the bottom. Once again, cloak and select your Speargun, then walk out slowly (being careful not to fall into the molten lead) and snipe the guy on the other side of the area. Now, wait until you've got about half your field charge, then cloak (assuming you've de-cloaked) and select your Shoulder Cannon. I'd recommend you save if you can right now. Then, run round the corner and zap the marine there. Switch back to electrical view, select the pistol and hit the switch. Turn around and destroy the alien there. Follow the corridor round, then when the Predator ship arrives attempt to jump onto it (wait until it stops in front of you). End of level.