Orvieto’s position atop a rocky outcropping made it a perfect redoubt in time of siege, easy to defend but with one big drawback—a lack of water. When Pope Clement VII decided to hole up in Orvieto in 1527 to avoid turbulence in Rome, he hired Antonio Sangallo the Younger to dig a new well. Sangallo’s design was unique: He dug a shaft 53m (175-ft.) deep and 14m (45-ft.) wide, accessible via a pair of wide spiral staircases that form a double helix and are lit by 72 internal windows. Mule-drawn carts could descend on one ramp and come back up the other without colliding. You can climb down, too, though it’s a trek up and down 496 steps, and there’s nothing to see at the bottom but, well, a well. A few steps up and down will introduce you to the concept, and give you time to contemplate the name. It’s a reference to St. Patrick’s Purgatory, a pilgrimage site in Ireland where Christ allegedly showed St. Patrick a cave and told him it was an entrance to hell.