The Cuevas de Jumandy are a network of underground river caves near Archidona, about 15 minutes north of Tena. You enter through a park with a swimming pool (popular with local families on weekends) and then follow a guide into the caves themselves, wading through waist-deep water in near-total darkness. The experience is somewhere between spelunking and swimming — bring clothes you do not mind getting soaked.
The caves extend several hundred metres and contain formations of stalactites and bats. A guided tour takes about an hour. The park charges a small entrance fee and guides are available on site. Go on a weekday to avoid crowds.