Start the chapter "Anticitizen One" (Its pretty late into the game) and get to the point where D.O.G. jumps onto the Combine Dropship. Head inside the hotel as usual, and grab the cardboard box in the demolished room. if you jump on it just right, you can get past the rubish blocking the stairs (Lazy Devs thought no-one could do it, i geuss), and climb to the top of the hotel. Take the box with you, as there are some impossible jumps without it. You can get all the way to the roof, and find that the sky is just as solid as you thought it was!