I believe I know where you are. Chapter 4, right? Here, watch this video, and tell me if anything is familiar. If I'm right, you should see your answer around 1:50.

http://www.metacafe.com/watch/6282199/po…

There should be a surface somewhere on the inside where you can place a portal, then, in the areas where you can see outside a little bit there should be a surface somewhere out in the distance, I got stuck here for a few mins as well, it was hard to see the surface outside, if I recall it was kind of a ways out.