From the start, cross the bridge, turn right and dive into the hole on the right, so you don't get crushed. After pushing the button, drop down underneath the block moving up and down, and into the side chamber. Timing is essential here. Or you'll have a sortof flat feeling.

Shoot the four numbers, one on each air duct. That activates a lift to a 'secret' goodie. When you reach the top floor, go behind the exit pod, there is the lift.