Most amazingly - you can swim in there! And if you're lucky, you can be there all alone for hours. Nirvana.
Wikipedia: cenote is a deep natural pit, or sinkhole, characteristic of Mexico and Central America, resulting from the collapse of limestone bedrock that exposes groundwater underneath. Especially associated with the Yucatán Peninsula, and some nearby Caribbean islands, cenotes were sometimes used by the ancient Maya for sacrificial offerings. The term derives from a word used by the low-land Yucatec Maya, to mean any location with accessible groundwater.