The Cave of Wonders is the centrepiece of Aracena and one of the finest cave systems in Spain. Twelve chambers and six underground lakes stretch beneath the castle hill, hung with formations of stalactites, stalagmites, and crystalline curtains built up over millions of years.
Guided tours (the only way in) last about an hour and follow a well-lit route through the main chambers, which bear names like the Naked Room, the Cathedral, and the Lake of Emerald. The cave is genuinely impressive — the scale of the formations and the clarity of the underground lakes make it a cut above the average show cave. Temperature inside is a constant 16°C, welcome in summer, brisk in winter. Book in advance at weekends and public holidays, when queues can be long. The entrance is below the castle hill on the main square.