"along the Colorado River in Black Canyon"

Clayton Sellers'voice echoes off the sheer rock walls that rise from his vantage point along the Colorado River in Black Canyon, just downstream from the base of Hoover Dam.

"[The dam] was started in 1931 and completed in 1935, about two years ahead of schedule and under budget," he says as he maneuvers a pontoon raft across the water. Sellers' passengers, mostly tourists staying in nearby Las Vegas, are awed by that fact, and even more so by the massive engineering marvel standing before them.

"It's about 45 feet thick on top. At the very bottom it's about 660 feet thick, which is like two American football fields and the end zones stuck end to end," the guide continues. "The concrete at the bottom has only been there about 75 years, so it's not all the way dry yet. If you go inside the dam, there's actually some seepage … through the wet layers of concrete."

Despite the still-curing concrete, the iconic dam stands as a testament to American ingenuity, built during the Great Depression by thousands of men who uprooted their families and traveled to the inhospitable desert in search of work. There were jobs aplenty, and after the huge task was finished, President Franklin Roosevelt arrived on Sept. 30, 1935, to see for himself — and to dedicate — the modern-day wonder.