As soon as you enter the level, switch to thermal vision (blue), as there are only marines on this level. Walk down the stairs and hit the switch by the last few steps. If you're on Training or Realistic, then feel free to explore the tunnels ahead, otherwise return to where you started the level. Jump on the rail at the top, then run along the rail and jump over to the ledge. Jump over once more to the third ledge and go through the door. Go up the stairs and up again, killing any marines who foolishly try to get in your way. When you go down the following stairs, cloak and switch to your Speargun or Shoulder Cannon. There is a guy with a grenade launcher on the same level as you who needs to be extinguished. When he's dead, drop down to the ground and proceed through into the tunnel ahead. The door on the left is locked, so go right down the stairs and kill any marines in the area, before pressing the button. When the video message finishes, head back up the stairs and through the door. You're in an area with water on the floor. On the right there's a door with a switch by it. The switch, obviously, opens the door. Hit said switch and enter said door, then go along the edge and round the bend. Make your way across the two bridges, and you'll enter a area with some gates, pass through or around them and enter the doorway in the opposite corner. Head up the stairs, along the walkway and down the steps. Hit the switch and go through the door. A bit further on, on the left, is a room with a switch. Enter and hit it. Then proceed through the door, and continue straight on (going left is pointless). You'll be in an area with a loader in it. Head for the door in the far left corner. Go down the stairs, then into the tunnel on your right. Hit the switch, go through the door and head down the stairs to find yourself at the waterfall. Go up the stairs, hit the switch and get on the lift. When it reaches the top kill the marine and press the button to open the airlock. Proceed into it to end the level