On an elevator in any building built by the forerunners (i.e. Assault on the Control Room) walk backwards into the control panel. You will start to kind of go up and down furiously before going strait through the panel.

When this happens, immediately walk forward, towards the lift. If all goes well, you'll sink halfway through the platform and be stuck. You can still look, aim, and shoot. You may have to try it a couple of times.

It's more fun when you play with a friend because you can activate the lift and see your legs flailing around from under the platform.