Surely one of the greatest triumphs of our understanding of the physical world - sending humans to the moon in the Apollo program of the late '60's and early '70's. A feat that would have been impossible without understanding gravity, Newton's laws, energy and momentum conservation, angular momentum, waves, the fundamental forces of nature.... (not to mention mastering engineering of all sorts, computer guidance, materials science, physiology, chemistry, etc!) Although we as a species have surely accomplished other feats more impressive (at some technical levels) landing on the moon perhaps had a symbolic value unequaled before or since. I hope we go back! (One of my favorite pictures from that era: earthrise as seen from the moon, from the National Air and Space Museum's web page.)