Cenotes are the result of freshwater dissolving limestone along weaken fractures in the rock, creating large underground caverns (uber simple explanation, true). Since the water table can be very close to the surface, the roofs of these caverns were often fragile, and collapsed - creating the sinkhole. Many Cenotes are connected via a labyrinth of underground passages and rivers creating a massive network of open and underwater caverns that cave divers treasure. If you go to the Yucatan and you get sick of all the bootie on the beach and sweating while you climb all over the Mayan Pyramids, you can always strap scuba gear on and explore the labyrinthine world of the Cenotes. They're pretty awsome.
What I didn't know: If you map the location of the 3000 some known Cenotes, they form a distinct ring. The center point of this ring is just off the shore of the peninsula. This ring is theorized to be the remnants of the Chicxulub Crater, a massive, 180-300km in diameter, impact crater - the remnant of the event that contributed to the massive extinction at the end of the Cretaceous period (the infamous extinction of the dinosaurs). If you were watching the cool TV program, they had some pretty amazing graphics that showed the near liquefaction of the earth's crust in the vicinity of the impact - though it doesn't really explain how this might have lead to the birth of the Cenotes. There's some world-renowned expert PhD mumbling about 'instability on the crater wall' and 'fractures in the limestone layer'...but no kick ass digital graphics. I guess I have to take more geology classes before I can explain it all here, and considering the stellar results of my last return to academia, I don't see that happening any time soon.
See - cool, hu?
Adjacent cenotes in the northeastern Yucatan
The life-cycle of a cenote. Eventually they fill with debris and become dry pits filled with vegetation.
An early debris pile at the base of a younger cenote
What I learned researching this blog entry further: First, Iain Stewart, the crazy Scottsman who hosts "Earth: The Biography" is actually a scientists, not an actor. I have to give props to the folks at NatGeo for not having the series hosted by Ewan McGreggor - though I find Ewan MUCH sexier. Scientists need to have fun, too.
Ewan McGregor (obvious difference in sex appeal)
One of New Mexico's Cenotes...