Ruins of Heliopolis

 Heliopolis, located in modern Lebanon at the town of Balbec, was, at its peak in the 2nd century CE, the largest spiritual sanctuary in the classical world.  Its temples feature the largest stones ever quarried, moved and set in place by human endeavor; some weighing almost 1,000 tonnes. The city dwindled in importance as Christianity gradually took hold in the area in the subsequent centuries.  A series of devastating earthquakes did the rest.