Temple of Mercury (Puy de Dôme)
Orcines, France
The Temple of Mercury at Puy de Dôme is a Gallo-Roman trachyte temple built in the 2nd century at the summit of the lava dome. It replaced a 1st-century arkose temple on the same site, which was apparently too small to accommodate the many pilgrims who visited. Its remains were revealed by excavation campaigns in 1875 and at the turn of the century.
Archaeological site data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).