Quick Tips:

There are several different cenotes, each one a unique, self-contained world by itself and in some cases very different. Some are small holes in the ground, other ones are huge caverns good enough for snorkeling and scuba; some have fish others are lifeless but breathtaking. If you have time and budget try a different one each time.

Best Way To Get Around:

If you only want to snorkel, you can get the bus or the colectivo vans departing from Playa del Carmen all day long.

Word of caution: if you don't have the training, NEVER dive by yourself in the cave portion of a cavern (the one away from daylight is called a cave, while the front, within visual reach of daylight, is the cavern).

src="http://www.igougo.com/photos/journal_photos/JungleMobile.jpg"
style="width: 226px; height: 150px;" align="left"> You arrive at the
dive shop and get your equipment fitted, and they then take you in their
JungleMobile to the cenote entrance--they also have snorkeling tours,
but I found the scuba diving much more interesting but not as colorful
(they put colored lights in the shallow areas for snorkelers).

src="http://www.igougo.com/photos/journal_photos/HiddenWorlds.jpg"
style="width: 182px; height: 270px;" align="left">Inside the cenote,
you descend a metal stairway into a wood platform in the middle of the
cenote through a small hole in the jungle--you have to contort a little, but
once inside, you feel in a different world.

At the beginning, the water
temperature comes as a shock in contrast with the warm air on the
surface, and here the water temperature is constant all year long--75F--always cool. In cavern diving, you always follow a cave-certified guide and never leave
him/her, as he is your only way to get back without trouble. They carry
double tanks and extra batteries for their redundant dive lights. Here it's
extremely important to follow the rule of thirds (one third of your air
to get to the farthest point, another third to get back to the starting
point, and the other third as a reserve, never to be used under normal
circumstances).

The cavern is amazing--so huge and
full of rock formations, the stalagmites and stalactites can be really
long and thick or as fragile as cake decorations, so please do NOT touch
anything you see.

Diving in a cavern is almost
effortless; you are just hanging in the middle of the water and slowly
kicking, frog-style.

An incredible experience.

There are two rivers to pick, but it doesn't matter, as the end is at the same area, and both have almost the same things to experience along the route. In case you get bored, you can exit at a couple spots along the river, but anyway, you will have to walk barefoot to the end to recover your belongings.

You can do it once to refresh, but it lacks interesting things to see or do, so it would be pointless to go again.

The only reason I found to get into the river next time is to avoid walking all the way to the coast and to have somebody else carry my stuff there!

Cool idea, but not well-implemented.

Dzenots in Mayan are the sacred holes with freshwater that were considered so sacred by the original inhabitants of the region, because, for them, it was a communication with the Underworld where the Gods lived, and it was also the only source of freshwater, so it is not a big coincidence that lots of big city/states were built near a cenote to quench their thirst and their fears.

What is more special about the cenotes is that, because of the kind of topography and soil (limestone) in the Yucatan area, there is no chance for rivers to exist, so the only sources for drinking water and irrigation are the rain and cenotes.

A cenote is formed by underground filtrations of water over millions of years. When water finds a layer of nonporous rock, it makes deposits of water, which, with time, link to form an underground river, and later, a system of interlocked underground rivers. With the flow of all that water going into the ocean, there are some areas where the ceiling becomes eroded and weakens, resulting in the collapse of the roof of the cave. The resulting hole in the cave is what is called a cenote, which can be used as a well, but it is not exactly one.

When a cenote is close to the coast, and thus to saltwater, there is a weird phenomena when both kind of waters mix, forming an "oily"-appearing layer in between both. This is called an alhoclyne. When you snorkel or scuba dive through this layer, your visibility diminishes completely, and the only thing to do is keep going up or down, as the layer is not too thick.

As the majority of the underground river is not exposed to sunlight, the temperature keeps stable all year round and all day long, basically at 75°F.

For scuba diving the cenotes, you need a 3mm-thick wetsuit, because, in the water, you loose heat 25 times faster than on land. Maybe you will feel comfortable for a while, but you can get hypothermia in the long run.

These marvels of nature can be very picturesque, and the formations you see come from the era in which the water level dropped and the empty caves began to fill with the typical stalactite/stalagmites, which, depending on the minerals of each area, acquired different forms and thickness. You may also see in some areas the roots of the trees trying to reach the water, a very weird sight.

If you are not claustrophobic, give them a try, as it is an amazing experience.