JUST HOW MUCH AIR DID I HAVE LEFT I pondered this rather significant question in the inky blackness, as I fumbled to get a safety spool out of my pocket.
I tried to remember the topography of the area I was in before I lost my buddy, and my third and last light had failed.
I continued feeling around to find something, anything, onto which to tie off my line, so that when I started groping for the main line that would lead to the exit, I wouldnt get even more lost than I already was.
Seven years earlier, I had arrived on the Rivera Maya in Mexico via wholly different means of transport. On that occasion I was completing an eight-day charity bike ride across the peninsula.
On the final leg, we had stopped at a local snorkelling spot called Grand Cenote. I remember diving into the crystal water and watching a team of divers disappear between the bleached white columns into the darkness beyond. It was this experience, coupled with the guarantee of warm, shallow water, that had brought me back to explore this, perhaps the strangest facet of our sport - cave-diving.
So here I was, five minutes along the main coastal highway from the popular resort of Puerta Venturas, at the small but exceptionally well-equipped Zero Gravity. I was with Fred Devos, one of the three very accomplished cave explorers who own the business. For five days, he would be my instructor.
Fred, originally from Canada, I can only describe as an enigma. The initial impression is of a very friendly hippie, with a low-energy, mildly detached persona. His outward appearance, does not match up with his existence, however. He has quietly got on and treated life as one long adventure, backpacking around the world, teaching English in India and exploring thousands of feet of virgin cave.
As if to highlight this mismatch of character and deed, once, on the way to drop off our cylinders after a long days diving, he suddenly pulled off the busy coastal highway to cut some flowers he had seen - with a metre-long machete!

AS WE WORKED THROUGH the course, Fred further confirmed that a calm exterior doesnt mean a slack mind. He didnt miss the smallest detail (even, disconcertingly, when it was pitch black). Everything was fed back into the comprehensive debriefs that followed every dive.
Considered the ideal size by Global Underwater Explorers (GUE), the Cave 1 course is usually taught to teams of three, but this time there were only two of us, the course minimum. My partner for the week was Zdenek Dusek, who was already a cave guide down the coast in Tulum, but looking to expand his knowledge. Zdenek was not native to the Yucatan but from the Czech Republic, where most of his early diving had been done, British-style, in cold, low-viz lakes.
We started with a morning in the shops small classroom for a large chunk of theory - cave-diving history and hydrology. Fred checked out our equipment and introduced me to some new, cave-specific things, such as the pigs tail.
This is a short length of shock cord secured to a bolt snap on which directional markers (arrows) and non-directional markers (cookies) are stored. I would be using these navigational aids extensively later in the course.
I was also introduced to burntime. In a lightless environment, your light is a critical piece of equipment. Knowing how long it will last is essential, so all batteries had to be checked daily.
Cenote Eden, one of the thousands of sinkholes on the Yucatan peninsula, was the site for our first days diving. It was just a few minutes from Zero Gravity; in fact the shop is only minutes away from all the main sites, with a half-hour drive considered a serious outing.
As we submerged for our cave-equivalent of a check dive, I realised that I could easily spend a whole day simply moseying around this cenote, looking at the catfish, eels and other critters that lived there. In Cenote Car Wash, which Id visit later in the week, there is even a resident alligator!
Fred had taken charge of the line and navigation on this first dive, but after a quick debrief on the surface, it was our turn to take command of the reel.
We tied off the line in the open water, and headed into a different passage called River Run for what was my first real cave-dive.

IT WAS A LEARNER CAVE; bigger and less delicate than many passages, but it was still overwhelming. Its main feature was a halocline near the ceiling, looking for all the world like a huge river flowing above us - hence the name.
This barrier, where salt and fresh water meet, is caused by the disparity in temperature and density. Its an amazing phenomenon. Passing through it is like swimming into a mirror, and can be more hazardous than it looks.
At one point, the guideline we followed ran just a metre below the halocline, so our fins, slightly above us in the cave-diving trim, were mixing it up.
I was following Zdenek, and suddenly it was like swimming in a mirage, impossible to see anything but blurred shapes, and certainly not the guideline.
It was shocking how easily we had gone from a serene swim with 100m of visibility [Really Ed. Maybe more, Chris] to vague outlines and no line!
This came up in the debrief and, once understood, was fairly easily managed.
Day two saw us back at Cenote Eden, and assuming a pattern that would ensue before each dive. A new skill or emergency procedure would be demonstrated, then extensively practised on dry land before further practise in open water.
This drill would be added to Freds list, and we could expect a simulated problem to beset our team at some point during the dive (usually on the way out)!
Before the failures started to hit, we were usually allowed to enjoy the breathtaking stalagmites, stalactites and icing-like flowstone, all exaggerated further by the bleached white of the limestone walls, which reflected light incredibly.

GUE COURSES INCLUDE A MANDATORY fitness test, where participants must complete a 275m swim in less than 14 minutes and be able to travel 15m on a breath-hold. Fred did all of this with us.
The test was undertaken not in a pool but in the early morning at Cenote Eden. With only birdsong for company, it was like swimming through a video promoting Zen Buddhism.
Understandably, skills around line-laying played a significant part on the course. Im always amazed how distracting a reel can be, transporting the user to a world were only you and it exist, and making communication very difficult. Enter the lights.
Passive light communication is a cornerstone of this course, with light movement being used to track both position and state of the rest of the team.
Rather than physically watching each other, as one might in open water, the light becomes the visual link between partners.
The following diver parks his light in front of his team-mate, and so remains in constant contact. Signals, such as circles for OK or a repeated sweeping motion for attention, are used, with rapidity of movement conveying the level of urgency.
Its amazing how sensitive you can become to this. As the days wore on, it was possible to deduce not only what was wanted but even Zdeneks anxiety level, purely from seeing his light.

AMONG THE LIMITS OF CAVE 1 are navigational constraints, a whole new topic for me.
Cave 1 divers are not trained to bridge between the permanent main line and other lines (an activity known as a jump) but they can navigate a T.
As the name suggests, this is a junction where the team has a choice of direction. It is critical to find the right way back, so we learned that it is good navigation practice to place a non-directional personal marker (often referred to as a cookie) on the exit side of the junction. Then, even if visibility has been lost, its possible to identify the right route out.
Youd be amazed how taxing such a simple task can be at first while trying to monitor your buddy, keep the light still, hold your depth, not disturb the silt, monitor gas and keep sight of the line.
In fact, the whole five days was packed with new information like this, which had to be absorbed before yet another layer of critical knowledge was stacked on top.
Each simulated scenario we undertook was designed to make us apply this new learning methodically, until it became habit. And it was for this reason that I was now in a cave system known as Taj Mahal, in total darkness with no line, no buddy and feeling, if not happy, then at least OK, because I now knew what to do next.
My time with Fred had highlighted that the walls of the Incident Pit are very much steeper in a cave, and I wouldnt need to break many rules to get into such a situation.
However, he had also helped me to see that correct behaviour and strict adherence to well-practised procedures would virtually ensure safety.
Course over, I stayed on and did some guided dives with Fred, which was a mixed experience.
I got the use of his vast knowledge of the area, some added experience and saw more spectacular environments. However, its hard to be completely relaxed, when your dive partner is the same guy who has spent the last week critiquing your every move!

Cave-diving are two words that more often than not elicit a sharp intake of breath from the other party, swiftly followed by a comment along the lines of you wouldnt catch me doing that.
In our daily lives, however, we are frequently just one step away from expediting our own demise.
So comfortable are we with the dangers and the procedures that keep us safe (like not stepping off a curb without looking), we no longer worry about them.
The course definitely qualified as advanced, and was one of the most taxing I have ever undertaken, but
I always felt safe, with each training experience challenging rather than stressing me.
Post-course, I felt confident that I could safely visit some of the spectacular and unique environments offered by the Yucatan aquifers, and for that reason alone it was one of the best training investments I have ever made.

To join the five-day GUE Cave Diver Level 1 (Cave 1) course, divers must have passed the GUE Fundamentals class at a Tec level, be over 18, a non-smoker and have at least 75 dives past OWD. The course includes a minimum 40 hours of instruction and at least 12 dives (eight beyond the daylight zone) at three or more locations. The price of $2200 includes park entry fees, national tax, cylinders, weights and nitrox fills. Playa del Carmen is 10 minutes from the resort of Puerta Venturas, www.dir-mexico.com