Concierge.com's insider take:

The Yucatán is one of the flattest places on earth—an area with almost no peaks or hills to accent its tropical terrain. As a result, the region's heavy rain collect in a series of unique underground sink wells called cenotes, which emerge as vast subterranean lakes and rivers. Many cenotes are located close to Cancún and make for unusual underground adventure opportunities (hiking, swimming, and snorkeling). Although there are literally hundreds of cenotes throughout the region, Hidden Worlds is one of the safest, and there are guided tours. It's located about 90 minutes south of Cancún and makes a great day trip if paired with the Tulum ruins (984-877-8535; www.hiddenworlds.com.mx; info@hiddenworlds.com.mx).