Cenote is a large natural deep pool of water fed from underground springs in the rain forest/jungle that is exactly round in shape. There are many in the Cancun area. There is an "Eco" combo tour that includes a bus trip to the Ek balum(means tiger or panther), ruin, part of which you CAN climb up on, a small native snack, and a swim at a cenote pool, and also a mountain bicycle ride (way too hot, the day I took the trip!!!) I did not have to go underground to swim in that pool. This time I will take the Chichen Iza trip... It too is an all day one...