One kilometer south of the Vía Blanca, near the airport turnoff, is the freshwater Cueva Saturno, a highly popular (read: crowded) subterranean cave with a pool billed as a snorkeling and/or swimming spot. The water's about 20°C and the maximum depth is 22m, though there are shallower parts. There's a snack bar and equipment rental post on-site.
