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L4D Version: 188.8.131.52
IHM Version: 1.1
Just after the waterfall and before the train tracks, down a passage to the left where there are 2 lifts.
The right lift is small enough to crouch under, and if you jump from here, you fall down to your death (intentional). However, you can get stuck on the left side of this shaft, with no way of climbing back up.