OUR CLOTHES WERE SODDEN; OUR SWEAT RAN FREE. This was some contrast to the wintry, flood-ridden UK we had left less than 24 hours previously.
     We were carrying a cylinder apiece; our Mayan helpers moved nimbly with two each. Ahead our host, Gunnar Wagner, slowly picked his way along the barely discernible trail. A German by birth, he and his Mexican wife Lina run the most prominent dive centre in Tulum, called Aktun. When not busy with tours, and cavern and cave-diving training, he goes exploring.
     Gunnars machete swung from side to side, severing the creepers that threatened to trip an ill-placed foot. Leaves and other organic debris concealed razor-sharp rocks and small potholes.
     We knew that snakes, scorpions and spiders needed to be treated with respect but, as we soon discovered, it was the unrelenting mosquitoes and ants which inflicted the most discomfort beneath the Mexican jungle canopy.
     Some 25 minutes and a mile from the roadhead, we reached our goal. Amid the entwined trees and creepers, a fair-sized walk-in entrance appeared. Local knowledge is crucial to finding such places for the first time, as even from the air they are impossible to spot.
     This cave, or cenote, soon to be named Dos Pisos or Two Floors, had been visited only once before by Gunnar and his friend Robbie Schmittner. Given its location, more than five miles from the sea, and the prospect of diving in any number of different directions, this was to be the site of our joint expedition.

cobalt water
The Yucatan Peninsula consists of 73,000 square miles of jungle-covered limestone, and there must be tens of thousands of such caves still awaiting discovery. Gunnar and Robbie had also made a preliminary visit to a second cenote some 600m distant. About 800m of line had been laid at each cave.
     It seemed reasonable to presume that they formed part of a single cave network. Preliminary investigations revealed a spacious arterial tunnel weaving and threading its way beneath the jungle, brimming with pure white stalactites and stalagmites. The waterway was gin-clear and, for the most part, the depth proved amazingly shallow at 7m!
     To the side, dark shadows and cobalt water clearly indicated a complex maze, typical of caves in the area. Where the main tunnel originated, and where the strong flow of water disappeared to, was unknown. The Mayan landowner, Roberto Canche, hoped one day to open the site to fee-paying visitors - a normal charge for entering cenotes is around US $5 . We wanted to explore, map and photograph it first.
     Our five-man British contingent would follow local convention in laying 1.5mm nylon line, pre-marked with small knots at 3m intervals. This would allow us to map the complex. To assist with this, we also had GPS, a laptop computer and specialist software which would speed the process.
     For much of the time we would operate solo, to cover as much ground as possible. The water was clear and warm at 25ÂC; thin wetsuits with no hoods or gloves were perfectly adequate for dives of a couple of hours or more.
     Shallow diving, typically at less than 20m, was a bonus, but by far the greatest attraction of such a project was the privilege of diving in tunnels so lavishly adorned with stalactites and stalagmites, between which we had to pick our way carefully. Dos Pisos was festooned with brilliant white formations throughout, and to state that these Yucatan caves are the most beautiful in the world is no over-reaction.
     However, in such inviting conditions it is tempting to swim well clear of the line and risk losing sight of it. Apart from the formations, the passageways are frequently floored with a cream-coloured sediment, so without constant line-awareness, the thin white cord could disappear, white against white.
     In certain areas, the sediment could rain down from above through percolation caused by our air bubbles, reducing the visibility as dramatically as any downblast occasioned by a careless finstroke.

cats and bats
Mapping the various passages turned out to be less straightforward than we had expected. We eventually overcame our difficulties in pin-pointing where precisely the various side leads departed from the original line by placing black cable-ties around the main line every 30m along the tunnel.
     Like a spider starting on an intricate web, the cave map slowly took form.
     Other features of the cave also impressed themselves on us. On the very first day Phil, our computer whizz-kid, discovered ancient Mayan pottery just a short distance from the entrance. This was photographed but most importantly, as required by law, not disturbed in any way.
     The cave fauna also proved interesting. Dark brown catfish about 20cm long were common in some areas and seemed remarkably unperturbed by our presence. These sightings were normally below small pockets of dry cave where fruit bats lived. Here a layer of black, marble-sized husks or nuts, presumably the discarded remains of bat-food, carpeted the underlying cave floor.
     In one remote side tunnel, a very distinctive, blind cavefish was discovered. We saw it only once, but it was 10cm long and eel-like. It resembled Lucifuga speleotesi, a species discovered in the Bahamas in the late 1970s.
     By the end of the second day, we realised that it would be harder to break new ground than we had thought. Depositing a partially used stage cylinder halfway along, we continued to the end of the 800m main line.
     From the map and the predominant direction of the cave, we knew we needed to find a passage trending to the left, yet 45 minutes in, we were being forced into a much smaller tunnel trending slowly right.
     In the cramped surroundings, heavy percolation and natural anxiety clouded our progress.
     Some 60m of new line was laid which, when surveyed and processed on the computer later, revealed that we had been following a tunnel which doubled back on itself!
     Acquiring a feel for the environment clearly takes time. On retracing our route, however, other leads did materialise and were marked for the future.
     A complex picture began to evolve over the next few days, but in simple terms, the Dos Pisos system was like the trunk and branches of a tree.
     In the upper sector of the cave, the various branches sub-divided to present progressively smaller tunnels, but downstream the main tunnel usually became larger as the various side-leads converged with the main outflow. Trying to follow the cave downstream therefore sounded a reasonable proposition.

two ways in
Nothing, however, is straightforward in cave exploration. Near the main cave entrance, the water current was very confusing, seeming to split so that the flow was hardly perceptible. The divers who combed this area described something of a maze on at least two different levels.
     A very large tunnel led steeply downwards close to the entrance, passing through a halocline from the fresh water into a clear but heavily silted saltwater passage, which appeared to terminate at a depth of 30m.
     Pat and Steve eventually narrowed the field in the shallow-water levels and Steve passed a slight restriction to regain a substantial ongoing tunnel, where he left his reel ready for the next dive.
     Ironically, in the neighbouring cave system, Robbie progressed upstream into the main flow the next day and discovered the reel! Unsure where he was, he recovered the object and exited along a route of which he was sure, unaware that Steve would shortly be looking for his lost equipment. The two caves had been connected, which was very satisfying in itself, and the length of the overall system jumped to more than 3.5km.
     Now the main emphasis of exploration lay in the terminal downstream section of the system.
     The tunnel here should get bigger as the water moved towards the sea, but instead, another extremely complex area was entered, with very low, wide and extremely heavy silting.
     Robbie and Steve bulldozed forwards in a blackout and the tension was heightened further when one of Steves second stages failed after being fouled by crystalline debris from the floor.
     We had all considered this potential risk, but in such confinement and with thin, easily lost line, it was a particularly nightmarish experience. More lessons were learned the hard way.

jungle commuters
After 10 days commuting to and from the jungle bases, we withdrew. The Dos Pisos system is now more than 3.85 km long, with every likelihood of considerably more passage being found. We had more than doubled the length of the known cave and conducted a comprehensive study of the network.
     Some of the dives had to run to three and a half hours in duration, and each and every operation reaffirmed the glorious nature of Mexican cave-diving.
     Now we understood why people like Gunnar had never been drawn to dive back in Europe. These magnificent caves represent the ultimate cave-diving on earth, and they just go on and on.

the Yucatan 2000 dive team, from left: Phil Dotchon, Dig Hastilow, Pat Cronin and Steve Marsh, Martyn Farr is kneeling at the front
Mayan pot found at 8m in the system
Line laying in Dos Pisos - white on white
Looking out from the huge entrance of Carwash Cenote
More views of Dos Pisos
Roberto Canche is the Mayan owner of Dos Pisos Cenote
Dive base