The Mayans considered cenotes to be sacred sources of fresh water, naming them “dzonot” meaning “sacred well” or “deep abyss”.
For some time, very long ago, the whole Peninsula of Yucatan was under water. The ocean covered the land and the land itself presented coral formations. When ocean levels dropped, all coral fossilized and turned into limestone. This is why there are neither mountains nor rivers in the Riviera Maya.
Limestone is a very porous material; water filters through, thus creating a whole underground rivers system, one of the largest in the World. When some areas collapse, they give entrance to the fresh water river as sinkholes. Dripping drops of water carrying limestone particles created amazing rock structures: stalactites, stalagmites, columns, waterfalls, kisses...
All cenotes are different from one another even though their waters are connected, like fingerprints, each one is part of a larger system.
Almost all cenotes offer private tours, as well as regular tours or you can just buy the entrance ticket and snorkel on you own.