Often lauded as one of the best-kept secrets on the Yucatan Peninsula, Rio Secreto (or Secret River) is an underground river and collection of caves. The bright turquoise water contrasts with the caramel- and mocha-colored rock formations within the caves, which include stalactites and stalagmites. Visitors can swim in the river, climb through the caves and enjoy the natural beauty of the area.

Recent travelers said this adventure was the highlight of their trip to Playa del Carmen, noting that the tour was fascinating and that they learned a lot about the Mayans and the land's natural history. Many also said this is a great rainy day activity since most of the exploration occurs underground. One gripe among reviewers: Visitors are not allowed to bring their cameras. Though a staff photographer is following you around and you have the option to purchase photos at the end of the tour, travelers said the photos are overpriced. If you don't have a wetsuit or water shoes, you can rent them through the company that runs the attraction.

Rio Secreto is open from 8 a.m. to 3 p.m. daily. Tours are available directly through the attraction's website and through other vendors like Viator and Playa del Carmen Tours; prices vary by company. Rio Secreto is located about 6 miles southwest of Playa del Carmen. Some packages include transportation. For additional information and background, see the Rio Secreto website.