In Mill Run, Pennsylvania.
At the famous Fallingwater, by Frank Lloyd Wright. A good glimpse at how he built the house over and around a natural waterfall, just one of the few ways he incorporated nature into the most beautiful house design in the country. Check out his Taliesin West near Scottsdale, Arizona, for another example of the same approach in a completely different kind of geography. Quite a couple of sights to see.
32squared 