Deep in the hill country near the remote village of Genga, the Grotte di Frasassi is one of Europe’s largest cave systems. This karst wonderland, gouged out by the river Sentino and discovered by a team of climbers in September 1971, can be explored on a 75-minute guided tour which takes in features such as the Ancona Abyss, a cavernous 200m-high, 180m-long chamber, and the so-called Gran Canyon full of parallel stalactites resembling pipe organs and waxy stalagmites that rise up like melted candles.
Tours in English depart at 11.40am, 1.40pm and 3.40pm daily from mid-May to June, with an additional 5.15pm departure in July and August, but it's worth calling ahead to check times. Outside of these months, tours are in Italian (with the exception of pre-reserved groups of 10 or more people). English-language audioguides are available, though these require earphones (either bring your own or buy them at the site for €3).
Wear comfortable shoes and bring an extra layer of clothing, as the 14°C temperature can feel nippy in summer.
The scale of the caves is quite astonishing. The Ancona Abyss, for example, would comfortably accommodate Milan Cathedral and contains forests of stalactites and giant stalagmites that reach lengths of up to 20m and took 1.4 million years to form. Other notable features include Niagara, a petrified cascade of pure calcite, and a crystallized lake.
To reach the caves from Ancona, take the SS76 off the A14 and look for the Genga-Sassoferrato exit. The car park, 1.5km east of the cave entrance at San Vittore Terme, is where you buy your tickets and catch the free shuttle bus to the caves. The closest train station, Genga-San Vittore Terme, is also next to the car park and ticket office.