Without it there would be no Las Vegas. As if to pay homage, most visitors take a day off from the casinos or the Strip to check out this man-made behemoth 35 miles from the glitz of Vegas. What amazes most is its size: Straddling the Colorado River, creating the border between Nevada and Arizona, this engineering feat, built in 1937, boggles the mind. Drive across Hoover Dam to ogle the precipitous gorge on one side and the limpid waters of Lake Mead on the other. Walk inside it to learn how it was built. And splash, boat or fish beside it in the lake for a day well spent ensconced in nature.