Millions of years ago, the peninsula of Yucatán was covered by the ocean; today, its singular calcareous soil consists of fossilized coral and former ocean floor. The entire area's lack of aboveground rivers is duly compensated by the existence of a network of underground rivers, caves and caverns that are fed by rainwater, which filters through the limestone and then flows towards the sea.

During the ice age, most of the underground cave systems dried up and the water that dripped through the porous limestone created the most extraordinary rock formations, which boast an incredible series of stalactites and stalagmites. When the underground rivers began flowing again, parts of the limestone vaults eroded, weakened and finally collapsed, unveiling a series of fresh water sinkholes. These were called "cenotes" by the ancient Maya, who considered them the entrance into the spiritual underworld.

It is recommended to take your own snorkeling or divig gear and insect repellent.

Visits the following Cenotes:

• Hidden Worlds (Dos ojos y Dos Palmas)
• Aktun Ha
• Gran Cenote
• Cristal Escondido o Maya Blu
• D'zinup

Cenotes are usually private-owned, so an entrance fee must be paid. Most offer rustic bathrooms, as well as parking; they close at 5pm.