Part 9Pipes, crabs, and secrets. It's as sexy as it sounds.
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In Chapter 1-D, after you come back from saving Olivia, there is a small room in far left section of the area after the elevator. (It's actually where you came from to get to Olivia.) If you crouch and walk underneath it, you encounter a dead scientist with a headcrab latched onto his face. There's not a whole lot that it does besides wiggle a little bit, but we haven't seen the last of this guy. [ 3 ]
The warp pipes on the other hand serve an actual purpose. In Chapter 1-B, you see the Civic Center for the third and final time, overlooking it from the catwalk. If you jump on the railing bit and jump towards the hotel bar, you won't make it. But you will make it if you activate a cheat to get one of your psychic powers, Timeshift . Doing a forward dodge and then using that will just barely let you snag the edge of the bar, and in the center is a pipe. Beyond that pipe are three more pipes, a la Super Mario Bros.
From left to right, the pipes take you to Chapter (or World) 4, 3, and 2.
EDIT: Added video for the real way to get to them.
These secrets don't seem to be very well known, as I haven't been able to find anyone else who has posted a video of either on Youtube.