Deep in the Parco Regionale del Monte Cucco is one of Europe's most spectacular limestone caves, with 30km of galleries reaching depths of 900m. Those up for a challenge can delve into its underground forest of stalactites and stalagmites on a guided two- to three-hour 'discovery' tour. For more expert cavers, there's a longer four- to five-hour 'crossing' tour (per adult/reduced €20/18, pre-booking necessary). Times and dates vary – check the website for an up-to-date calendar.
For more details, stop by the Grotta's info point in the nearby village of Costacciaro (Via Valentini 31).