This is a spectacular cave, about 1km long, featuring numerous marble caverns, soaring high and hollow like great cathedrals, connected by a fresh stream. The caverns are guarded by shrill, shrieking flocks of guacharos (batlike nocturnal birds). You can only visit on a guided tour; you’ll be given a life vest and be expected to wade across the river. A headlamp and waterproof shoes with good traction are hugely helpful.