Cenotes of the Yucatán Peninsula in Mexico

Cenotes are natural pits or sinkholes resulting from the collapse of limestone bedrock that exposes groundwater underneath.

They are especially associated with the Yucatán Peninsula of Mexico, which is primarily made up of porous limestone.

For millions of years, rainfall slowly ate away at the limestone and a huge system of underground caves and caverns was formed. 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 Yucatán Peninsula.

Ref - Cenotes of the Yucatán Peninsula in Mexico