The Coves de Sant Josep (Caves of Saint Joseph) in La Vall d´Uxió (Castellón)

The Coves de Sant Josep (Caves of Saint Joseph) are situated at 30 km from Castellón de la Plana.
According to its official website, this cave has the longest navigable underground river in Europe. You can also visit it by kayak.
The cave is located in the vicinity of the town.
The following links may be of interest.

 Coves de Sant Josep: This is the oficial website in English.