(Re)discovered in 1932, the Grotte di Pertosa-Auletta date back 35 million years. Used by the Greeks and Romans as places of worship, the caves burrow for some 2500m, with long underground passages and lofty grottoes filled with stalagmites and stalactites. The first part of the tour is a boat (or raft) ride on the river; you disembark just before the waterfall (phew!) and continue on foot for around 800m, surrounded by marvellous rock formations and luminous crystal accretions.
This grotto complex is quite commercial, with souvenir shops, bars and a €3 parking fee. It is located 6km north of Polla just outside the northeastern corner of the national park.