The dome that crowns Florence’s great cathedral of Santa Maria del Fiore—the Duomo—is a towering masterpiece of Renaissance ingenuity and an enduring source of mystery. Still the largest masonry dome on earth after more than six centuries, it is taller than the Statue of Liberty and weighs as much as an average cruise ship. Historians and engineers have long debated how its secretive architect, Filippo Brunelleschi, managed to keep the dome perfectly aligned and symmetrical as the sides rose and converged toward the center, 40 stories above the cathedral floor. His laborers toiled without safety nets, applying novel, untried methods. Over 4 million bricks might collapse at any moment—and we still don’t understand how Brunelleschi prevented it. To test the latest theories, a team of U.S. master bricklayers will help build a unique experimental model Duomo using period techniques. Will it stay intact during the final precarious stages of closing over the top of the dome?