According to tradition, the Scala Santa was the staircase from Pilate's palace in Jerusalem—and, therefore, one trodden by Christ himself. St. Helena, Emperor Constantine's mother, brought the 28 marble steps to Rome in 326. As they have for centuries, pilgrims still come to climb the steps on their knees. At the top, they can get a glimpse of the Sancta Sanctorum (Holy of Holies), the richly decorated private chapel of the popes containing an image of Christ "not made by human hands." You can sneak a peek, too, by taking one of the (non-sanctified) staircases on either side.
Piazza San Giovanni in Laterano, Rome, 00184, Italy