Beneath the town's castle hill is a web of caves and tunnels carved from the karstic topography. An extraordinary 1km route takes you through 12 chambers and past six underground lakes, all beautifully illuminated and filled with weird and wonderful rock formations, which provided a backdrop for the film Journey to the Center of the Earth .
Tours are in Spanish (audio guides available) and depart according to demand. Tickets can sell out in the afternoons and on weekends when busloads of visitors arrive.
