Beneath Aracena's castle hill is a web of caves and tunnels carved from the karstic topography. An extraordinary 1.2km, 50-minute loop 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, with optional English-language audioguides available for €1.50. Frequency of departures varies according to demand. Tickets can sell out in the afternoons and on weekends when busloads of visitors arrive.