The Sewers: Pumped
Follow the tunnel until you come to a small passageway to the right. Drop down into the room and prepare for a series of firefights with a SWAT team. There are two SWAT team members in each of the next three rooms. Note that the barrels scattered about will explode if shot, and any seasoned action gamer knows how enemies love to stand around explosive barrels. Go into first-person mode and target the barrels to clear the rooms quickly.
You'll emerge in a large room filled with machinery and, unfortunately, more SWAT members. Luckily, there are more exploding barrels in here as well, so use them. You'll need to clear the room, then look for a red ladder near the hallway from which you entered this area. Climb the ladder, and follow the walkway around, then focus-jump to the next platform. The passage ahead is blocked by steam.
Turn around and take care of the newly arrived SWAT team, then jump down to the ground and go through the gate by which they entered. Drop down the hole, and run through the tunnels until you see a SWAT team member behind a chain-link fence. Shoot him through the fence, then climb onto the table to your right and hop up to the ledge.
More police are waiting for you. Continue through the hallways into the large chamber beyond. As expected, more SWAT. Once the room is clear, find the pump with the red blinking light. Turn off the pump and another SWAT team arrives. Once you're done with them, proceed to the next room and climb up the ladder.
The Sewers: Ice and Corrupt
Two allies are in the next area, being beset by police. Take out all the enemies in the room, then jump up onto the ledge near the entrance. There's a small ledge by a nearby gear that you can climb onto, so do so. A cutscene will show Ghost falling into the level below.
Take out the police, then head to the left. Continue through the passageways, making sure to grab ammo from the police you'll encounter. Just before the ladder back to the upper level, you'll run into a guard with grenades. Grab his grenades if you need some, then climb up the ladder.
Again, Ice and Corrupt are being attacked. Clear the lower level of the room, then climb up onto the ledge with the machine that is spinning wildly out of control. Ignore the SWAT team members that come into the room--it's a never-ending wave, and so you're just wasting ammo. Go into first-person mode and target the three large anchors at the base of the machine. Destroy all three, and Ice and Corrupt can escape. Do a focus jump into the nearby hole to proceed to the next level.