millions of tourists flock to Pisa, Italy, each year to see the monumental eight-story tower that seems to teeter precariously—and miraculously, without falling—in the Piazza del Duomo.But it’s no miracle that the bell tower—built as a companion to a cathedral—is still standing. It’s due to multiple feats of engineering that may preserve the Tower of Pisa’s lean at a precise angle for potentially centuries to come.