One of Rome's four great pilgrimage churches was built on the spot where a 3rd-century pope witnessed a miraculous midsummer snowfall (which is reenacted every August 15). The gleaming mosaics on the arch in front of the main altar date from the 5th century. The apse mosaic dates from the 13th century, and the opulently carved wood ceiling is believed to have been gilded with the first gold brought from the New World. To view the elaborate 14th-century facade mosaics, inquire at the souvenir shop. Daily 7 AM-8 PM.