Ponte Sant' Angelo, Rome, Italy

Ponte Sant' Angelo, Rome, Italy

Ponte Sant'Angelo spanning the Tiber in Rome, one of the eight stone bridges the Romans are known to have built over the Tiber between 200 B.C. and A.D. 260, is the most celebrated of the six "massive beauties" still in use, said Judith Dupré, author of Bridges. "The Romans perfected the masonry arch," she said, allowing them to span much greater distances than previously. "Much of Roman engineering genius is underwater, hidden from view, but their inventions—including the cofferdam, cutwater piers that divide water current, and pozzolano, a type of waterproof concrete—are still used today," Dupré said. Ponte Sant' Angelo, originally named for Hadrian, the emperor who reigned during its construction, leads to his mausoleum, Castel Sant' Angelo, a popular tourist attraction in Rome.