Temple of Mercury, Baiae, Italy - Baiae was an ancient Roman beach resort town fueled by hot springs the town was a pleasure retreat for the wealthy Romans and Emperors. The city was sacked in the 5th century, abandoned in the 15th century, and slowly deteriorated and sank into the sea due to volcanic activity (it's near Pompeii and the infamous Mt. Vesuvius).
Pompeii, Italy: Temple of Isis. The Cult of Isis was a mystery religion in Rome. The temple was walled off and only initiates were allowed inside. The rituals and ceremonies that took place inside are unknown.