On Volcán Villarrica [1]’s southern slopes, but outside Parque Nacional Villarrica [2], the misleadingly named “volcanic caves” are in fact lava tubes that descend 340 meters into the mountainside (unless you’re a bat, the darkness at the bottom is absolute). Unfortunately, what was once a professional introduction with bilingual guides to volcanism, including displays on volcanic geology and seismic technology (ranging from a classic seismograph to modern computer monitors), has declined and is difficult to recommend with any enthusiasm.
Reached by a fork off the road to Parque Nacional Villarrica, the Cuevas Volcánicas (tel. 045/442002, cuevasvolcanicas [at] hotmail [dot] com) are 14.5 kilometers south of the main Villarrica–Pucón highway. Some of the well-trained guides can manage English if necessary; admission and the one-hour tour cost US$17 per person for adults, US$10 per person for children.
Links:
[1] http://www.moon.com/destinations/chile/sur-chico/pucon/sights-and-recreation/parque-nacional-villarrica/volcan-villarrica
[2] http://www.moon.com/destinations/chile/sur-chico/pucon/sights-and-recreation/parque-nacional-villarrica