What major 18th-century discovery stimulated the european fascination with classical antiquity?

   Herculaneum, Pompeii

Explanation:  These are ancient cities of the Roman Empire, covered with ashes and rocks after the eruption of Vesuvius 79 AD. They remained preserved because of ashes, carbon and other materials from the eruption, and were discovered during the 18th century. Herculaneum was better preserved (almost intact), when the interest of Europeans for classical antiquities, as well as for the culture and art of the classical age in general, began. It was a period characterised by neoclassicism during the mentioned 18th century. The appetite for classical works of art and antiquities has grown so much that young men from higher social classes had a cultural tour as part of their education, visiting well-known sites with archeological remains from the classical period.