Gamma Energy visits the Aperture Science East facility, built after GLaDOS gained sentience. The goal was to create a facility where GLaDOS' research could be examined and tested without harm to employees or other people who actually... mattered, without her knowledge. Pick up a prototype Portal Device and try not to die.
This is the hardest chamber in the game, and was featured in the second trailer. I have yet to beat it myself without using the bullet time cheat. My brother has, though. He's better at this kind of stuff than I am.