The cenotes are underground rivers and run throughout this area. They were formed when the meteor that wiped out the dinosaurs hit here. There are some small fish that live near the openings where there is some light, basically places where the roof fell in. Deeper there are some small blind fish. Nothing else, it's limestone so really alkaline water and without light nothing lives. There are huge caverns completely underwater and the water is so clear that you can't see it so diving in it is like flying. Enormous stalactites and stalagmites that were formed when the caves were drier during an ice age. In fact, I've seen black walls and charcoal from old fires as well as small handprints on the walls, all from when the caves were dry from the ice. Completely underwater now but once they were dry. It's magical and beautiful and can be very dangerous as well.
The Maya consider the openings to be entrances to their version of hell and you will usually find small altars hidden away near the entrance to pacify the Alux who are tricky spirits who can make things miserable. The Maya also say that they came from the caves, the first men. During some difficult times in the past Maya have hidden in the caves for years. Their creation myths mostly center around the caves and the heroes and rulers of the underworld.
Many of the cenotes are open for tourist tours and are really beautiful, tropical, jungle paradise.
