Cueva de Montesinos is a cave of the Province of Albacete, Spain. At the bottom of the cave is a small lake formed by rainwater filtering through the cave. It has been discovered through a series of experiments and tests that the water in that lagoon is connected by underground streams with the natural park of Ruidera.
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An episode involving descent into the cave is included in Part II of Miguel de Cervantes's novel Don Quixote.