Beneath the Préhisto Parc, 16km southwest of Porrentruy, lie these stalagmite-filled caves, discovered in 1886 and open to visitors by guided tour; dress warm (the caves are freezing). A combined ticket for the caves and Préhisto Parc costs Sfr15/10 per adult/child. No credit cards.
To get here, take bus 78 from Porrentruy station in the direction of Fahy and get off at stop Chevenez, place la Grangette, where you'll need to change to bus 73, bound for Damvant and alight at stop Réclère, Les Grottes (Sfr13.20, 40 minutes). Otherwise, rent a car and drive.