Your new objective is to find a way out of the sewers. You'll find yourself in a some sort of sewer system. Keep going down and down, there's no other way anyway. To make things easier, you may want to switch to night vision. Eventually, you'll reach the actual sewer tunnel, with two paths to go, left or right.The path on the right is collapsed, so take the left, brighter one. As soon as you can, turn right and follow the tunnel until you reach a smaller sewer. Crawl through it.