Cenotes are natural pits or sinkholes resulting from the collapse of limestone bedrock that exposes groundwater underneath.
They are especially associated with the Yucatan Peninsula of Mexico, which is primarily made up of porous limestone.
Many filled with water from rain or from the underground water table.
When the roof of a water filled cave collapses, a cenote is born.
There are an estimated 7,000 cenotes in the Yucatan Peninsula.
Second, cenotes were also important for religious reasons.
They believed cenotes to be portals to the underworld and a way to communicate with the gods.
Archaeological research has found evidence of religious ceremonies that took place in or around cenotes, including human sacrifices.
Some cenotes have been turned into public swimming pools of sorts.
One of the best examples is the Cenote Zaci, located in Valladolid.
Another cenote with some tourist infrastructure is the Cenote San Ignacio, in Chochola.
Finally, the facilities at Cenote Sambula, in Motul, were recently remodeled.
Sources:Wikipedia,Yucatan Holidays,Merida