Catania has been a prize of many empires over the centuries, from Greeks to Romans to Arabs to Normans to Spaniards (to name a few). But its citizens have a more dangerous enemy right in their backyard—Mount Etna, Europe's largest and most active volcano, which destroyed the city with earthquakes and lava flows in 1693. Look closely at the baroque buildings dating from after the eruption—you'll notice a creative use of lava.