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.
I feel like I missed some opportunities in the initial release with the caves and water. Chamber 25 Advanced includes a number of techniques dropped from the initial release for both the "test chamber" area and the "behind the scenes" area. You may recall that the solution to Chamber 25 included a short trip behind the test chamber to flip a single switch conveniently located very close to the test chamber. As that was only the fourth chamber made for the mod, I hadn't yet mastered some of the finer points of BTS areas, and mostly wanted to shy away from them. The advanced version of this chamber which is coming with the One Point One update includes a rather long and complex journey into the massive drainage and cooling system that a large underground mountain research facility might need.