Metropolis Magazine invited structural engineers Leslie Robertson and Craig Schwitter to pick their favorite feats of man-made wonder. One of their picks is Machu Picchu, the mysterious ruins built by the Incas.
Built on the steep slopes at the crest of the Andes Mountains, with terraced scientific and agricultural areas, Machu Picchu features wonderful, almost jointless stonework. The Incas were superb craftsmen and thoughtful architect-engineers. They had a sublime sense of how to create an opening through a stone retaining wall. They sloped the stones toward the opening, capping it with a lintel, but why they never used the arch is very strange. Was this because of some religious or cultural belief? Certainly it was not for lack of engineering talent.
Take a peek at the rest of their picks: http://www.metropolismag.com/cda/story.php?artid=2470 - via Kottke
But I wish I'd seen the Gateway Arch on their list. That is one amazing piece of architecture. I guess it helps that I also have a sentimental attachment to it as well... ;-)