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Cave Diving in the Yucatan.

Oliver C. Wells

The idea of seeing stalactites and 'mites underwater has been something of an obsession with me since seeing photographs of them in Robert Palmer's book "The Blue Holes of the Bahamas."  I suppose it was inevitable that I should go for a cave diving holiday in the Yucatan peninsula (May 3-10, 1991).

I spent the week with my wife Pamela and six cave divers in the cenotes within a few miles of Akumal, which is about 100 km south from Cancun.  The sea level was about 400 feet lower than it is now during the last Ice Age, rising to about the present level between 18,000 and 6,000 years ago.  During the Ice Age these caves were dry and 'tites and 'mites were formed in great numbers.

I found myself to be totally unprepared for how wonderful these caves are.  Imagine a flat country with impenetrable jungle on both sides of narrow, straight roads.  Here and there an unpaved road (if you can call it such) or a path leads off into the jungle to a cenote, which is a pool of water typically 50 yards across and 20 feet deep.  There may be an upstream cave underwater at one end and a tunnel downstream at the other (who knows? JGC).

Everything is highly organised in cave diving these days.  At the entry level is the "cavern diver" who stays within sight of daylight at all times.  Typically this involves a large underwater cave entrance sloping down to 50 feet depth, and with large stalactites on the roof.  An impressive underwater notice in two languages asks that people with no cave diving experience should go no further than that point.

Permanent nylon lines with a diameter of perhaps 0.125" start at some distance inside the dark zone. The dive leader lays a line from a belay point (plus backup) in the open water of the cenote to the near end of the permanent line in the dark zone in the cave, and reels it in again when it is time to leave.  The "main line" goes in along a chosen passage with triangular plastic arrows at intervals to the closest exit to air (there are numerous entrances and exits in most caves there).  In other words, these arrows either may or may not help you to come out again by the same route as you went in.  Lines into side passages start at various distances from the main line and can be reached with a "gap reel" if you know where they are. "Decoy lines" guide visitors away from places where the 'tites and 'mites are especially fragile.  The passage size inside the cave might be twenty feet wide and high (sometimes larger, sometimes smaller) at depths generally between 50 and 70 feet.

The halocline is at a depth of about 50 feet, where the fresh water lies on top of salt water. Initially the interface is quite sharp, but it widens to a few feet after divers have gone through.  Within the mixed zone vision is blurred.  The most curious thing is the way in which you must let air out of your buoyancy compensator (BC) as you go down through the halocline, and put some in when you come up.  This is as opposed to the normal situation when you put more air into your BC from time to time as you go down and let it out again as you come up. (You progressively lose buoyancy with depth in water of constant density because the air bubbles in your wet suit are compressed.)

The other divers wore the standard Florida cave diving rig with twin back-mounted 80 cubic foot tanks and a high capacity BC between the diver and the tanks.  They did not wear helmets, and one diver did not wear a hood (the water was quite warm).  Dive lights were in the range from 30 to 50 watts with the battery on the waist strap. Reels, dive computers, backup flashlights and other items were attached appropriately.

I was surprised by the arrangement of the manifolds.  The two tanks were permanently connected together with a separate valve for each of the two regulators. In other words, if the rupture disk on the tank blows out, then the air supply is gone.  I mentioned that I had seen a diver lose all of his air in this way in an open water dive a few years ago, but this information was not too cheerfully received. One of the divers told me that he preferred a manifold with one regulator on each tank and with an equalizing valve between the two.

I had two side-mounted 80 cubic foot cylinders with a separate regulator and pressure gauge on each of them.  The use of totally independent respirators avoids the problem mentioned above, but does not give access to the contents of a cylinder if a regulator should go wrong. The reason for doing this (as recommended by the Cave Diving Group) is that while carrying two completely independent bottles and regulators does not give access to air in a failed system while the diver is underwater (between sumps, yes!) it does however leave adequate air in the remaining working system for a return to safety if 'the rule of thirds' is used correctly during the whole dive.  (I describe this below.)

Obviously I am not intending to criticize anyone in this article.  Redundant respirators of equal size were first suggested, I believe, by Michel Letrone in 1955, and have been developed in various forms since that time. They are in use widely in the North-eastern USA, for example.  On the subject of rupture disks, Billy Young writes: "Your concern over loss of air from blow-out disks is one that cave divers have overcome by 'double-disk' installation.  This raises the safety factor considerably."

My greatest difficulty was in swimming on the surface of the water across the cenote to the entrance of the cave.  The other divers with their high capacity BC's and no weight on the head floated cheerfully with head and shoulders out of the water.  I had a small BC on the chest and the weight of four flashlights on the helmet (two primary plus two backup).  The result was that I floated with my mouth about level with the surface of the water.  On the second day I took my snorkel tube and the problem was solved.

Entering the water was a bit of an adventure at times.  For example at the Temple of Doom, a hole in the ground about twenty feet across gives access through the roof into a chamber containing water about 15 feet deep. The approved method of entry is to jump in.  In the event, I found the THUMP on arriving at the water surface after a free fall of 12 feet with two side-mounted 80's to be more violent than I had expected, but survivable.  (Cave divers have been known to enter the water from even greater heights than this.) A wooden ladder was provided to get out again.

My wife Pamela came for a swim at the Temple of Doom, being nibbled by the little green fishes when she was in the water and bitten by insects when on the ladder (there were very few biting insects anywhere else).  The instructor told us that in some of the caves these fishes had learnt to follow a diver into the dark zone where they eat the defenceless animals that live there.

On the checkout dive I swam slowly admiring the view, showed the instructor when I changed to the second mouthpiece at two-thirds pressure on the first tank, and called the dive when I was equally down on the second.  It was becoming clear that my objectives were different from those of the other divers.  I was there strictly as a tourist to admire the 'tites and 'mites and I had no interest in going too far from the cenote if I could possibly avoid it.  The other divers had studied the cave surveys and had decided to visit the more distant points.  In addition to the disparity in objectives.  I did not have one of my regulators on a long hose, which is considered to be essential for helping your companion by the Florida divers.

The other divers were courteous and helpful but it was clear that our aims were not compatible.  We solved this problem in the obvious way. My companions swam along the line at their own speed and vanished in the distance while I operated solo between them and the entrance.  Typically I would swim slowly for half an hour or so at depths between 50 and 70 feet through the wonderfully clear water and large chambers, admiring the 'tites and 'mites at leisure, being quite enchanted by these places.  My procedure was to swim in until I reached thirds, swim out again to the warning notice, recalculate thirds, swim in a second time, and so on.

I am not worried by the idea of solo cave diving.  This is a decision that divers must make for themselves.  You go into a different mindset being more careful about everything and being much more willing to stop where you are for minutes at a time if this is necessary to consider a question that might have arisen.  I shall not tell you how long I stopped at the line junctions and at some of the more intricate belay points, checking the arrow out, looking along the lines this way and that way, and examining the situation until I was sure that I could find my way back to the cenote even if all five of my lights had failed.  (The "arrow out" may or may not indicate the way that you actually went in and this can cause difficulties if you overlook the fact.  In the event the instructor explained the layout of the lines and the other divers held detailed discussions of the line junctions also. This is a serious matter.)

My final dive was in "Carwash,” so called because cars were washed there in the past (but fortunately not now).  This was unusual in having algae in the top six feet which was therefore a brightly illuminated light green colour with a visibility of about six inches.  You hit colder (but still quite warm) clear water below this where you can see for tens of yards below a bright green ceiling. By this time my companions had gone on ahead, so I swam around in the massive cave entrance until I found their line, and then on in.

After five consecutive days of solo cave diving my breathing rate was less than half of what it had been on the first day without my having made any effort to improve it whatsoever. My buoyancy control was much better, and I had finally learnt how to swim with my feet high to avoid stirring the silt.  Removing my ankle weights had been helpful here.  I swam slowly admiring the view for about 700 feet to a place known as Luke's Hope where you can see a bright green glow from an air surface.

Luke's Hope was discovered by a diver who was lost and almost out of air, rather in the way that Bob Davies discovered Wookey Thirteen in December, 1955.  He surprised his friends by taking a taxi back to where they were staying and greeting them long after they had given up hope of ever seeing him again.

While I was looking up at the bright green glow from Luke's Hope I became aware that the newer of my two regulators was free-flowing.  Not very rapidly, but an unwanted bubble every two seconds or so certainly clears the mind.  There was no line to the inviting green glow up above and I had no idea whether it would be possible to get out at that point.  I would have had to lay a line from my reel if I had wanted to investigate that matter.  On the other hand, I still had 2,000 psi in each cylinder, both regulators were otherwise working properly and the free-flow was nominal.  This was one of those occasions when I stayed where I was for several minutes to decide what to do.  At one point on the way out the delinquent regulator started to bubble away quite merrily, so I gave it a knock and it settled down to its previous slow pace. Back at the warning notice it stopped free-flowing altogether.  I also dived twice in Maya Blue and once at Naharon.

In our non-diving time we visited the Mayan ruins at Tulum and at X-Caret.  Many thanks to Steve Gerrard for organising the diving and for lending us his car on our day off, to Tony and Nancy DeRosa and Shelley Baker for the other arrangements, to my fellow divers and to my wife Pamela who has put up with this sort of nonsense for so many years.

I would like to thank Peter Schulz, Kevin Wills, James Coke, J. Billy Young and Michael Madden for their comments on early drafts of the above, and JGC for his more detailed comments given below.  If you are qualified as a cave diver, then you should not miss a visit to these caves should the opportunity arise.

Comments on the above by James G. Coke IV of the Akumal Dive Shop, Playa del Carmen, Quintana Roo, Mexico 77710:

(1)     Luke Boissoneault made his historic dive on Nov. 21, 1985 in the company of two advanced open water students and an open water instructor.  They planned to reach a slate attached to the permanent line about 400 feet into the upstream section of Carwash.  I had explored that region originally 3 or 4 months prior.  The team of 4 (NOT trained in Cave Diving) reached their goal and signed the slate.  I still have this slate in my possession.  Silt flew at this point and three divers exited in confusion while Luke went exploring in the wrong direction where no one had been before. As he ran out of air he found the hole bearing his name; got out, cried and thanked God; and walked out of the jungle carrying his equipment!  He caught a cab to his camp ground where his initial hellos to the assembled group were treated like the salutations of an apparition!  I still keep in touch with Luke; he is a SCUBA instructor living in Quebec.

The other three divers also had their problems. One of them ran out of air shedding his equipment in the cave.  Moments later his guide found him (after assisting the other out of the cave) and brought him out.  He was resuscitated at the surface.

(2)     The "little green fishes" that follow divers into the cave are Mexican Tetras (Astyanax mexicanus).  They are found in almost every cenote in this region. There are some colour differences between the fish occasionally, from cenote to cenote.  They are very aggressive and will take advantage of a cave diver's lights when looking for small and large troglodytes to eat.  They will attack and devour the smallest to largest animals, including Typhliasina pearsi (a blind fish that occupies these caves).

I have seen Tetras as far back as 2,500 feet from air and sunlight; either lost fish or ones who have followed me in. BOLD little gaffers!!  This behaviour appears to be confined to the cenotes that are the most popular.  Carwash, being the most popular site, has the biggest problem with them.  Temple of Doom, Maya Blue, Naharon etc. are less popular, so fewer fish follow a diver into the cave.  So on the whole, the problem becomes more widespread as the site becomes more popular!

Tetras cannot follow divers through the halocline because the lack of oxygen in the lower salt water kills them. Amphipods and Isopods make short work of their carcasses should they remain in the lower salt water for too long.

(3)     There are areas of certain caves that have been declared sensitive; therefore divers have been asked not to visit these areas if they are not engaged in a worthy study of sorts.  For example, less-than-perfect buoyancy control and bubble damage from open circuit SCUBA plays havoc with fragile soda straw formations! Nobody likes to be told that they are not wanted, but what else can we do?

(4)     The underwater warning signs in Spanish were donated by the Cave Diving Section of the NSS.

(5)     The halocline is shallower in caves that are closer to the ocean.  There is a lens of moving fresh water laying on a static base of salt water, basically. At The Temple of Doom (TOD) (4.5 km from the ocean) the halocline is at 50 feet; the Carwash halocline (11 km from the ocean) is at 65 feet.  Often the interface is sharp; however there is a mix zone in some caves where the fresh water flow strips water from the lower zone.  Current-deficient areas such as in the Madonna passage in TOD display a sharp interface.  A Line Maya shows a distinct mix zone.  I am studying these phenomena with U of New Orleans currently.

(6)     The wooden ladder out of the water at TOD was installed by Mike Madden.  He also maintains the guidelines at Nohoch Nan Chich.  He installed the TOD cavern circuit guideline.  I maintain the guidelines in Carwash, Naharon and Maya Blue. All of the above have been surveyed (except TOD) with maps published; except for Maya which will be in print by the end of this year (46,600 feet of passage surveyed to this date with the Maya Naharon system --- Sistema Naranjal).  Many others have explored in these caves also.

(7)     The algae/tannic bloom in the top few feet in Carwash only occurs in the summer months.  In winter, the entire pool is clear.  The bloom is a by-product of the man-made clearing around the cenote. The natural sponge of the jungle has been removed; now all the rainwater washes tannic into Carwash.  Five years ago the cenote was clear all year round.