An engineering wonder about 30 miles southeast of Las Vegas, this dam supplies 20 million residents of California, Nevada, and Arizona with water and provides hydroelectricity to 1.3 million customers. No written description truly captures the visceral effect of peering out over the parapet at the top of the dam and looking down at the Colorado River, racing along 700 feet below. More than 1,200 feet wide at the top, this all-concrete wonder was the largest dam in the world when it was completed in 1936 (it was one of many public works projects intended to help lift the country out of the Great Depression). A bypass bridge opened in 2010 and provides fabulous photo opportunities for pedestrians. Experts lead walking tours that will thrill future engineers. One tip: Avoid visiting in the summer when temperatures can hit 110 degrees. 866/730-9097, usbr.gov.Tour Adults, $11, Kids 4-16, $9.
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