"Thousands of years ago, the entire Yucatán Peninsula was under water, as evidenced by its massive network of rivers that flow beneath the region’s limestone surface. No place better presents the area’s captivating caverns and underground water systems than majestic Río Secreto, a nature park just south of Playa del Carmen that offers adventurous travelers wide-ranging cave tours. Extending for miles, its river system wasn’t discovered until 2006, when a local man accidentally stumbled across an entrance while chasing an iguana through the jungle. When you go, guides lead you through a maze of stalactites and stalagmites that ends with a swim in subterranean waterways that vary based on the path you choose."