The largest masonry dome on earth, Florence's cathedral of Santa Maria del Fiore, is a masterpiece of ingenuity and a source of mystery. Historians and engineers have long debated how its architect, kept the dome perfectly aligned and symmetrical as the sides rose and converged toward the center. To test the latest theories, a team of U.S. bricklayers will help build an experimental, mini-Duomo, using period tools and techniques.