Head toward the back of the Main Altar to see a true wonder originally designed to solve a lighting problem. This is the Transparente, a Baroque extravaganza of marble, bronze, and floating angels. In 1732, Narciso Tomé actually cut a hole in the ceiling to let a beam of sunlight fall directly onto the central tabernacle, making the stone appear to glow. It is theater in architecture. While you are marveling at the sights, keep in mind a practical pointer: this is a sacred space, so please ensure your shoulders and knees are covered as part of the mandatory dress code. Afterward, don’t miss the Sacristy. It is effectively one of the world's finest art galleries, housing El Greco’s famous painting, The Disrobing of Christ, which has hung in that very room since 1579.