I was on a flight from dfw to las; the route almost always passes over Hoover dam, which if you sit on the left side of the plane, gives you a spectacular view. The pilot on this trip noted that we were passing over Boulder Dam. According to the las history channel, available in most las hotel rooms, the dam (built in the early 30's) was first called Boulder Dam, then renamed Hoover Dam, renamed Boulder Dam and somewhere in the late 40's congress officially (and permenantly) named it Hoover Dam. His calling it Boulder Dam is interesting, since that name was only used for a period of over 60 years ago.