Cenotes: The Sacred Waters of the Riviera Maya

From an article on Playa Maya News:

The natural wonders of the Yucatan Peninsula are countless, but some of the most unique to the area are the cenotes. Cenotes are created by an underground river system and are fresh water sink holes that the Maya considered to be sacred. In addition they were an incredibly important resource as a fresh water source, and the Mayans also believed they were the entrance to the underworld. Cenote, (say-NOH-tay) called dzonot (ZO-note) by the ancient Maya were defined by the Motul dictionary, a dictionary of Mayan hieroglyphics, as "abysmal and deep" or "hole filled with water".

Millions of years ago, the Yucatan Peninsula was covered by the ocean. Some 15,000 years ago, during the last Ice Age, the sea level descended approximately 250 feet.

For thousands of years, the porous land surface, formed by fossilized coral and limestone, has filtered rainwater, which dissolved parts of the subsoil. This process created a system formed by flooded underground rivers and caves. This phenomenon is truly unique, and makes up the largest network of caverns in the world.

Cenotes are formed when the roof of a cavern collapses due to erosion. The level of the water also contributes to the creation of cenotes: if it is too low, it does not provide enough support, which causes the roof to weaken and cave in.

The depth of each cenote depends of the amount of natural debris that has accumulated through erosion in addition to the remains of the roof that collapsed. The water that gathers in these amazing natural wonders is a crystal clear turquoise color with a very pleasant temperature of 78°.