Caves of Sant Josep Nature · La Vall d'Uixó, La Plana Baixa, Castellón Caves in La Vall d'Uixó containing the longest navigable underground river in Europe, 2,750 metres long, 800 of which can be visited by boat. Photo: Simon Burchell (CC BY-SA 4.0) What to see and do in La Vall d'Uixó