Los Espejillos has several waterfalls and natural swimming pools, with lovely, clean and refreshing water sparkling over the polished black rock that characterizes the area. It stands across the Río Pira’i 18km north of the highway. The falls are only accessible in dry weather (November is usually the best month); the roads become dangerous when it rains.
It's best to book a tour from Santa Cruz. Otherwise you can get take the bus to El Torno, where you can look for a 4WD to take you up, though finding one is not guaranteed. Expect to pay around B$100.