So, a few weeks ago I was in sunny Mexico (well, sunny for the most part, except for the one and a bit days of rain….but who cares when you’re underwater anyway).
Specifically, I was on the Yucatan Peninsula, just outside of Playa Del Carmen (on the mainland across from Cozumel). While I did some dives out on Cozumel and a night dive off the beach of Playa Del Carmen, what blew my mind was cenote diving on the Peninsula itself.
What is a cenote? Well, a cenote is a sinkhole, often with exposed groundwater (meaning fresh water), that leads into an underground cave system. There are cave systems that we can dive all over the world, but the cenotes in Mexico are some of a rare breed.
The common understanding of how the cenotes were formed is that the sea level, many millions of years ago, was much much lower than it is today. In fact, there is even an argument that this drop in sea level occurred due to extensive global atmospheric upheavals that took place after a massive meteor crashed into the Yucatan Peninsula (also blamed for the decline of the dinosaurs). The resulting sea level drop left a large limestone landmass where the Yucatan Peninsula lies today.
Over millions of years, precipitation mixed with naturally occurring and increased carbon dioxide levels (due to a lack of substantial flora, necessary for ordinary localized photosynthesis), slowly dissolved large portions of the limestone, forming caves and subterranean channels beneath the Peninsula. Eventually, the sea levels began to rise once more and combined with the surface waters that were gradually filling the cave system, flooded the entire subterranean system.
In certain areas, where huge caves and channels formed underground, the domes of these caves and channels dissolved to such a degree that the ceilings were unable to support themselves and collapsed inwards. It is these openings that gave rise to cenotes and provided access to the local indigenous population to fresh water supplies.
This is a depiction of a typical cenote:
So, now you know where cenotes came from. But why are they cool?
For oh so many reasons. First, it is very rare to be able to dive in a cave system that was fully formed when it was still a dry system. Due to the surface water slowly working its way down to the bottom of the cave system in an effort to make its way to the ocean, it totally reformed the rock formations, often creating speleothems (of which the more commonly known ones are stalagmites, stalactites, columns and flowstones) were formed inside the caves. As a result, this is one of the few places on earth where you can dive in fresh water and see submerged “dry-land” formations.
Not only are there very cool speleothems, but also some awesome crystals that sparkle when your dive light passes over them. In some of the cenotes, these crystals create a mini-light show as you pass by some of the smaller passages. And if you are diving in a 65 million year old cave system that has speleothems and crystals and used to be dry, what else do you think you could expect? Fossils!! Some spectacular perfectly preserved fossils sitting there at 6 metres of fresh water.
Additionally, because the vast majority of the cenotes that are dived on the Yucatan Peninsula are in fairly close proximity to the surrounding Caribbean waters, there is often an underlying influx of salt water, with the fresh water from the land’s surface on top. This is known as a halocline, where there is a sharp change in salt concentration over a small change in depth. You can actually visibly see the change, as the salt water tends to result in a blurry effect where the two densities of water meet and mix. If you’ve ever seen oil and water mix, it’s a similar effect. If not, it’s like diving on a “trip” without needing to partake in any narcotics!
You can see the effects of the halocline in this excellent video from Advanced Diver Magazine:
A very neat effect of the halocline is also the “ripple” that is observable on the walls at the edges of the halocline. I don’t know the reason for this, but you can see specific lines of different water consistency that seem to flow down the cave walls. It’s weird, because you are underwater, but it almost looks as though there is water flowing down the cave wall in front of you. A small effect, but one that constantly had me staring agog at it.
Another side effect of diving in fresh underground water was the changes to one’s buoyancy. I ordinarily dive with 3-4 pounds of lead while diving salt water in a 5 mm full-length wetsuit. I effectively dived weight-free in the cenotes, even though I was still wearing my 5mm, plus my 5 mm hooded vest (those water are chilly)!!
Despite the saltwater content having a little less visibility, the distance one can see in the fresh water (provided you have a strong enough dive light) is unbelievable. I have never experienced such crystal clear waters. It was so much fun to look a far way off into the cave system and see what was coming up.
With great visibility also came fantastical light shows. One of the other most mesmerizing aspects of many of the cenotes is the change in water colour between the entrances to the cenotes and further in. My best memories were of sitting there in the cooler blue waters just inside the cavern entrance and then looking up out of the mouth of the cenote. You could often see a complete reflection of the tree line and the cenote mouth on the surface of the water
…. and then get a spectacular laser show as the sunlight streamed down in sharp beams of brilliant light to illuminate the green, brown and crystal clear blue waters.
This would always be augmented too by tiny mirrors or slivers of mercury above your head …. mirrors created as your rising air bubbles caught in small depressions on the ceilings of the cenotes.
Anyway. Diving the cenotes for me was a magical and stunningly surreal experience. Although there isn’t a huge abundance of marine life, it is the effects of the formations, the haloclines and everything else that make them a truly remarkable experience.
I only had a chance to dive a number of cenotes: Chac Mool (twice), Dos Ojos (twice), Chikin ha and Tajma-ha … and there are lots of them! I think another trip might be in order …