Cuba's largest cave system lies about 15 km southwest of Viñales village and can only be visited on a guided tour, which is the right way to see it — the scale of the passages requires a knowledgeable guide to navigate and interpret. The tour covers several kilometres of the cave, passing through chambers of impressive formations: stalactites, stalagmites, and the subterranean lakes that make the Viñales karst particularly rich.
Unlike Cueva del Indio, Santo Tomás is not motorised or heavily commercialised. The tour is on foot, with hard hats and carbide lamps (authentically old-fashioned) provided. Tours depart from the entrance at set times; book in advance in high season as group sizes are limited.