One of the stunning features of this structure is known as the oculus. This 30 foot wide opening is like an “eye” in the dome’s ceiling and is the only source of natural light for the building’s interior. Its marble floor is sloped so that when it rains, the water comes through the oculus and drains out through camouflaged holes along the base of the building. Completed in about 126 A.D., this ancient Roman temple was built to honor all of the gods of Rome. 500 years later, the emperor gave it to the Pope and it became a Catholic church, which spared it from being pillaged or destroyed during the Middle Ages.