Grotte di Castellana

The Grotte di Castellana is one of Italy's most spectacular cave systems, glittering with crystals and filled with weird and wondrous rock formations spread throughout a series of large caverns and long tunnels. The first great chamber is almost 200 feet deep, with a great oculus at the top open to the sunlight and rimmed with streamers of vegetation. Bring warm clothes for the 48º F temperature. The short, one-hour 1km (0.6 mile) tour gives you a taste for this underworld, but it's worthwhile to take the two-hour, 3km (1.8 miles) tour that includes the caverns' most stunning sight, the fabulous "Grotta Bianca" where the calciferous water seeping through has so few impurities that the crystalline stalactites and stalagmites it has formed shine a brilliant white.
You'd think that for €16 they'd let you take photographs—or at the very least throw in the parking, which costs an extra €2 The caves are open daily 8:30am to 7pm. The longer tour (in English) currently leaves at 11am and 4pm; the shorter English-language tour leaves at 1pm and 6:30pm.