Half-Life 2 Walkthrough -> Through the Channels
Be very careful on the slippery ground. There are more lickers (suckers) here; use a crowbar or, if you're tired of it, throw any barrel and run under them. Repeat the same trick with another batch of hanging "tongues." In the boarded-up room, you'll be met by a local.
Help him get rid of a new enemy—three Cutters. Collect supplies in the room and move on. The Cutters are easily shot from behind the door with the stretched mesh. Around the corner, take out your pistol and light the barrel on fire, then retreat. At the exit from the catacombs, shoot the soldiers, then retreat through the door to slowly deal with a whole swarm of Cutters.
Go down and lure the soldiers out. After that, calmly climb back up the stairs and shoot everyone as they appear. Now you have a submachine gun in your hands. Check the dark passage on the right for supplies and entertain the lone zombie.
As you emerge from the water, prepare your machine gun: you'll face a battle with Unification soldiers and a pair of cutters. In the heat of battle, it's easy to miss the tongue of a remora. Climb onto the pipe and, avoiding the remoras and jets of steam, move toward the "lambda" and the boarded-up passage.
Once out of the water, quietly and carefully take out the soldier next to the car with your pistol. When the barrels roll toward you, don't hesitate—detonate them immediately. After a short but intense battle, dive back into the catacombs, into the friendly embrace of the cutters.
There, without waiting for the cutters to visit, jump into the water, dive into the pipe, and emerge on the other side. A new problem has arisen – how to get to the opposite side of the room. It's too high above the water. You'll have to dive. At the very bottom, several boxes and spools are blocked with boards. Free them and use the "floats." Escape into the light of day.
After passing two more cutters, gather supplies and cross the makeshift plank bridge to the rebel cache. Jump down. The Geiger counter has started to work—I strongly advise against stepping on those green, oily puddles. Those who want to can take the pipe to the ammunition stash on the right. The rest of you, move forward.
Your pistol will be your best friend for the rest of the level. The settlement is infested with headcrabs, which the Combine is shelling the area with. It's best to shoot these creatures from a distance, dodging their jumps.
A few times, the headcrabs will dig themselves out of the ground. The zombies lying down are actually just resting. Be careful—a zombie will probably throw a barrel at you. Carefully avoid the dangling live wire. Don't touch the walls of the sparking train car.
Just beyond it, next to the bridge, another victim of the headcrabs is lurking in the radioactive sludge. Run along the bridges to the "garage," where you'll be provided with an amphibious vehicle.