Swamp

The Swamp is the second chapter of the campaign Swamp Fever in Left 4 Dead 2. It begins in a train underpass fashioned into a safe room for the first chapter. The Survivors temporarily touch ground, but then move on to planks resting over the water. Once the planks end, the group finds the ruins of a long abandoned boat house and a dock, which happens to be loaded with various supplies. Nearby, there is a plume of smoke above the trees. The path leads into a clearing with a crashed plane. Fortunately, the aircraft is filled with supplies. The crash has slowed the Survivors' progress, however, and they must open the emergency exit to proceed—which, of course, triggers a Crescendo Event. Once they have weathered the hordes of the Infected, the group makes their way through some ravaged old shacks and into the next safe house.

Trivia

 * The plane is similar to the one which crashed at the start of the Dead Air Finale.
 * Witches can spawn within the swamp completely submerged in water. Survivors can be easily attacked if they're not paying attention.