Jonathan Karl did something that very few civilians ever get to do: ride along on the U2 spy plane:
The U2 goes to such a high altitute you you basically have to wear a spacesuit. I'm doing what they call the 'pre-breathe' which is basically getting in the suit and breathing pure oxygen before you go up to the flight. The reason why you do that is they want to get all the nitrogen out of your blood because if nitrogen is in your blood when you reach the altitude of 65-70,000 feet, it will actually try to get out and your blood will literally boil.
ABC World News Webcast has the story: Link [Flash video]