About 400 years ago, a throng of dusty workmen laid down their shovels and huddled around an ancient painted wall -- a fresco, technically -- unearthed in a tunnel near Italy's Bay of Naples. The men were at work on a massive construction project, burrowing through a hill to build a canal for a local armament factory and mill. No one expected to find fine art. But as the workmen dug deeper into the hill, they encountered wonder upon wonder -- house walls painted blood red and sunflower yellow, fragments of carved inscriptions, pieces of Roman statues. The architect supervising...