I’M SINKING THROUGH the wispy milkiness, ghostlike and ethereal, my body disappearing in stages, being consumed by the 3m deep sulphur cloud. It looks for all the world like some scene from a vampire movie.
Huge tree branches, protruding, seemingly alive – moving, ready to grab or entangle feet or legs, as the cloud seeps around, with dancing tendrils like huge trails of cigarette smoke.
I am in a cenote called Angelita, or the Nightmare. Located 15 minutes south of Tulum, or a 90-minute drive from Cancun in Mexicos Yucatan Peninsula, this cenote is just one of an estimated 7000 natural sinkholes resulting from the collapse of limestone bedrock that exposes groundwater below.
The area is famed for its crystal-clear ocean, colourful healthy reef and large pelagics, especially whale sharks in season.
I had however chosen to forego these attractions after being sandwiched between two tropical storms, one from the Atlantic, the other from the Pacific. They had seemed to meet each other in a Mexican stand-off right over the peninsula. So I was now diving in the relative safety of the area’s wealth of inland underground caves.
The word cenote comes from the Mayan word dzonot, or sacred well. The cenote begins as an underground chamber that forms when the limestone substrate is eroded by the action of rainwater. As the chamber expands, there is less support from the water, so the roof often collapses, leaving underground water-filled caves.
Cenotes may fully collapse, creating open-water ponds, or collapse only partially, with rock formations hanging over the entrance to the cave systems.
The size of cenotes varies from small pools to 10s of metres in circumference. Most of those in the Yucatan Peninsula are small sheltered sites without visible surface water. Formed more than 6500 years ago, an extensive system of underground interconnections links many Mexican cenotes.
Over the past 20 years more than 300 miles of interconnected passageways, sinkholes and caves forming this incredible eco-system have been discovered by scuba-divers. From time to time, these systems find their way to the ocean, and sea water enters the sinkholes.
Being heavier than fresh water it sinks, and at certain depths where fresh and salt water mix, haloclines may be formed, causing interesting and unusual visual effects.
A change in temperature and buoyancy may also be experienced depending on the depth of the cenote and the amount of seawater present. Sea water is often warmer than fresh water, which can provide a pleasing transition.
In Angelita, a medium-sized round open sinkhole, the pit is very deep, at 60m. It is unusual in that the boundary between fresh and salt water at 32m mixes with the hydrogen sulphate emanating from rotting vegetation, causing an ethereal cloud.
Because of the two tropical storms and the accompanying days of torrential rain, the level of the water had risen by 2m, making the cloud level start at 34m. The rain had also caused the normally clear visibility, with sunbeams lighting the whole cenote down to the sulphur level, to reduce to 5-10m. It was almost as dark as a night dive.
We had approached the cenote along a 200m forest path after kitting up in the car park, making our slippery way to the edge of a large pond surrounded by a canopy of trees and vegetation.
I imagined how beautiful the sun-dappled water would be in perfect conditions, but as it was mud, rain and rotting leaves were making me wonder what we were doing.
Our dive guide Chucho (Jesus Guzman) of Koox Diving, located in the centre of Tulum town, explained that there would normally be a high drop of around 2-3m into the water.
However, as the water levels had risen so substantially, we were able to slip in, walrus-like, off a ledge, instead of having our tanks and cameras lowered to us on ropes.
The cool, 20° water shocked me at first. As I became accustomed to it, Chucho signalled for us to light our torches and descend. He had already given us a very thorough briefing in the relative warmth and dryness of the dive-shop, followed by a reminder in the car – he had insisted on driving us to the cenotes in his truck, to save our hire car from getting muddy.
At around 15m I looked up and was surprised, because of the depth of blackness, to see a glowing, emerald fog-like halo above us – the surface.
Having no perception of depth, I kept a constant check on my dive computer, and Chucho’s bright torch-beam.
At around 30m, he signalled for us to stop above a large leafless tree, its branches reaching out to us like fingers grasping for a handhold, with swirling mist only just visible below.
Our brief had been to descend to the cloud, penetrate it and come out below at around 37m, where we would investigate the tree roots and vegetation. Very careful not to get entangled in the branches, and watching our depths avidly, Chucho led us to one side of the sinkhole to search a small cave for tiny bat bones.
The smell of sulphur, which had escaped into my mask when I cleared it, was strong and distinctive.
After a few minutes we started to ascend back through the cloud, keeping our torches on each other.
We raised our heads out of it, our eyes by now more accustomed to the darkness. The eerie green glow above made our bottomless bodies resemble a scene from a Hitchcock movie.
Finally reaching the surface, we swam to the muddy edge of the pond and climbed out carefully.
Wary of the slippery rocks, we carried our gear back through the woods, fighting off huge mosquitoes biting our faces and ears, the water having washed away our repellant.

SHOWERING OFF THE SULPHUR STENCH from our wetsuits in the very basic facilities near the car park, Chucho decided to take us to a different cenote for our second dive.
There are thought be thousands in the Yucatan Peninsula, with only around 30% having been discovered so far, and only around 30 well-known to divers. The cenotes are owned by the state, but the land around them is mainly privately owned, so you normally pay a fee to access the water.
Many owners figure this to be a great source of income, charging $10-40pp entry. The larger, more popular sites, although more expensive, usually have better facilities, such as a basic bathroom and shower.
Driving through a sudden deluge of rain, dodging red-backed tarantulas, large puddles and huge potholes, we headed west towards Coba for 15 minutes or so.
Beside the road, and encroaching onto it after all the rain, was a series of linked ponds, in which Chucho told us several crocodiles lived. We were excited to try to look for them, but the ooding had caused them to retreat further into the forest.
This cenote, Akun Ta, had been renamed Car Wash by taxi-drivers who once used its waters to clean their cars. Chucho told us that this cenote was his favourite, and a very special place.
I could see that the water was an orangey-brown, caused by the rotting red mangroves and vegetation as well as minerals in the soil, and vastly exacerbated by the raised water level caused by tropical storms.
At different times of year, the cenote would vary in colour from crystal blue to emerald green to sludgy brown, and occasionally the colour it was that day.

AS WE APPROACHED THE POND, we saw again that the water levels had risen considerably.
Chucho, who had lived near there for many years, said he had never seen the level so high or the colour of the water so pronounced. It looked as if we were about to dive into a lake of Coca Cola.
The wooden steps into the pond were submerged, so we turned and sank backwards into the water.
At around 3m, we suddenly emerged into a crystal fairyland. Spinning around we could see the edges of the 50m-wide pond in all directions.
The bottom was at only 6m, and tall stems of lily-like plants stood sentinel around the whole circumference. Darting between the stems were small, brightly coloured fish, flashing greens and pinks as they caught our lights, resembling dancing fairies.
A soft, lush, emerald carpet of algae grew across the floor of the pond. Looking towards the surface, it was amazing how the brown turned bright orange as it reflected against the sky.
If it was like this in torrential rain, what must it be like in bright sunlight
Chucho led us over to a dark area I had spotted at one side of the pond.
It turned out to be the entrance to a cave system. He had briefed us that he would lead us into the caves, laying a line as he went.
Securing the first part of his line, we followed single-file, wary of our fins stirring up the bottom, as we descended through a cut in the rock edging the cenote.
Moving slowly, stopping every 10m to secure his line to another rock, we followed Chucho down to around 12m and through a series of caverns, twisting and turning this way and that.
Coming to a large wall, he motioned for us to come and look at a set of objects, including the pottery head of an animal – offerings to the Mayan gods from some long-ago age.
Passing stalagmites and stalactites, all too soon Chucho signalled for us to turn and slowly follow his line out. He brought up the rear, untying his line as he went.
Finishing the dive in the 3m shallow part of the pond, it was like being in some childhood dream of pixies and enchanted lands.
On an earlier trip in June, we visited two other cenotes in the area, Chac Mool and Kukulcan. In the far better conditions then, we experienced them in the normal way.
Diving the larger and deeper Chac Mool first, we entered through a small open pond. Cutting down under water through an overhanging cliff into a series of rooms, we passed several small roped-off tunnels.
We then came to a bigger tunnel with a large “danger” sign. Descending to 12m, we passed two levels of beautiful stalactites and stalagmites. On one ledge a holy statue had been placed.
On our way back to the entrance, we entered a huge room where we ascended slightly, coming closer to an opening in the roof where light rays speared the surface like lasers.
Fallen tree trunks and vegetation littered the bottom, sometimes protruding into the cavern. Seeing another diver swim past the entrance to the room revealed the true scale and magnicence of the cavern.
Following a short surface interval, we entered the second cenote, down a set of steps into an underground cave where the entrance to the water was hidden. The entry (and exit) was precarious, slippery and uneven.
Back-rolling into the small, shallow pool, narrow but elongated, I descended only a metre or two. I looked around, but couldn’t see where we were to go.
Chucho started to head down to a tiny cut in the rock, barely wide enough for him to fit through. Squeezing through the tunnel at around 10m, with my large camera, I was disconcerted to find the visibility turning to jelly as Chucho’s fins mixed the fresh with the sea water, causing a halocline.

AT THE OPENING INTO AN UNDERWATER room, I moved slightly above Chucho to the undisturbed water and followed him into a series of caves.
We entered a room where the roof had partially collapsed, leaving a domed air pocket. A large mineralised tree-root descended through the rock into the middle of the air pocket, ending around 1.5m under water.
Surfacing and removing our regs, we expected to breathe dank, old air, but it was incredibly fresh.
Heading back towards the entrance, we passed through a cavern where a bright green opening made an interesting contrast to the darkness of the cave. This was another entrance to the cenote.
I have heard people say that once you have seen one cenote, you’ve seen them all. But in fact every cenote I saw differed in colour, structure and visibility. Visiting at different times of year brings new characteristics caused by changing surrounding vegetation.
With names such as Dos Ojos, the Pit, Pet Cemetery, Dreamgate, Temple of Doom and Garden of Eden, there are many more cenotes for me to explore, and still more to be discovered. It is hard to grasp how large an area these hidden caves must cover, and what beauty lies like a secret waiting to be discovered by those who dare.

GETTING THERE Fly direct from London with BA or Virgin, or through most major US gateways, to Cancun. Tulum is a 1.5-2 hour car ride from there.
DIVING & ACCOMMODATION Koox Diving, www.cenote-diving-mexico.com. There are many large hotel resorts and small boutiques hotels around Tulum.
WHEN TO GO Cenotes can be dived year-round. For bright, sunny, clear conditions go from December-June. Hurricane season from June-November can bring heavy rain, reducing visibility but causing unusual colour and light effects.
MONEY US dollars are accepted everywhere, though you may get change in Mexican pesos. Be wary of filling stations if you hire a car. They will set their own unfavourable exchange rate for US dollars.
HEALTH No vaccinations or malaria tablets needed. Mosquitos can be a problem in rainy season, so take good insect repellant.
PRICES Koox Diving can arrange accommodation as well as a range of diving packages. A five-day, five-cenote pack with six day’ hotel stay (two sharing) would come to US $1200pp. A five-day stay with three cenote days and one sea day costs $885. Scheduled flights from around £600.
TOURIST INFORMATION www.visitmexico.com