These spectacular limestone caves, 40km southeast of Bari, are Italy's longest natural subterranean network. The interlinked galleries, first discovered in 1938, contain an incredible range of underground landscapes, with extraordinary stalactite and stalagmite formations – look out for the jellyfish, the bacon and the stocking. The highlight is the Grotta Bianca (White Grotto), an eerie alabaster cavern hung with stiletto-thin stalactites. 'Speleonights' take small torch-wielding groups into the caves after dark, among the bats, beetles, and crustacea that live there.
There are two tours in English: a 1km, 50-minute tour that doesn't include the Grotta Bianca (€12, on the half-hour); and a 3km, two-hour tour (€16, on the hour) that does include it. The staff like you to assemble in good time before your scheduled tour, and remember that temperatures inside the cave averages 18°C, so take a light jacket.
In the same complex, you'll also find a speleology museum and an observatory.
Grotte di Castellana can be reached by rail from Bari on the FSE Bari–Taranto train line (€3.20, 1¼ hours, roughly hourly).