Around the turn of the 20th century the United States had a problem. The young nation was growing, particularly the Southwest (Arizona, Southern California, Nevada) and particularity the farming and produce field. People needed water to drink, bathe in and grow crops.
It took the brilliance of over 200 engineering minds to come together along with Herbert Hoover's political wrangling for the Boulder Dam to open its gate in 1935. The dam was not only an engineering feat of massive proportions but a financial relief for thousands of out of work Americans during the Great Depression. You should spend time walking through the lobby display area to read the history of how this massive undertaking came to take place--including the brave men who literally risked their lives dangling from ropes to blast rock and sediment to make what would later be named the Hoover Dam come to life. Engineering buffs will love to learn the amazing attention to detail that went into making this place.
If you can afford the $30 you need to buy the full tour. Be aware that on weekends there will be a wait. This tour includes both the area of the generators (about a 30 minute tour) PLUS going deep underground the dam where you get to peek out one of the air vents built into the dam wall. If you want to see the 15 minute movie and walk around the area overlooking the dam the walk-through rate is $8. You have to pass through metal detectors before entering.
Not only is the Hoover Dam amazing because of the way it was built or the significance it still has today (Water from the Hoover Dam provides nourishment for 1/3 of the crops grown to feed the United States) but fans of Art Deco will appreciate all of the Art Deco details such as on the winged statues built to pay homage to the men killed that built the Hoover Dam and even on the elevator that brings you down the lower section of the dam.
Parking is $7 but you can walk to the Arizona border and have bragging rights. Just be aware that AZ doesn't recognize Daylight Savings time so in the fall you will be in a different time zone when you walk the couple of blocks over. Also, check out the Pat Tillman Memorial Bridge near-by which gives amazing views of the Hoover Dam from afar.