This cave is called Sac Be Ha.

I AM SITTING ON A BOARDWALK in the sun, and around my feet a shoal of small fish flit chaotically to and fro. The water is 25C, and the pressures of the world melt away.
We are not battling with traffic or rushing to catch the tide; we are savouring a unique environment.
To my right, a spacious low arch with airspace leads away into the darkness of the cave. To my left, the sun-drenched, rock-sided depression is a haven of tranquillity - for now, were alone.
This idyllic setting is Nohoch Nai Chich, which in Mayan translates as Giant Birdhouse. I have never been here before, but it has been in my mind for more than 25 years.
This place is legendary - and, like everything in the world, it has changed.
Helen, my partner, came here seven years ago. What an adventure that must have been! At the time it was the longest underwater cave in the world, and to get here the team had to hire horses to transport their equipment several kilometres along the rough jungle track.
Equipment was then lowered on ropes to the waters edge before embarking on a dive that epitomises the best of underwater Mexico.
Today we have the luxury of being able to drive our heavily laden car directly to the site. There is a very small community here living in white painted mud huts with grass, or natural palapa thatch-type roofs.
We have paid our access fee to a smiling, helpful Mayan, and from our arrival, a feeling of well-being settles on us. So often development is for the worse but here, far from the coastal strip with its rash of grandiose hotels and spreading golf courses, I get that rare feeling of wonder.
This place is an environmental success story. Its tidy, its clean; its being cared for and developed empathetically. We are here in the quiet period, November, at the tail-end of the hurricane season.
The chatter of voices heralds the approach of a party of Brits. Some eight people gather on the boardwalk, clad only in swimwear and life-vests. Equipped with masks and snorkels, they are given a good, friendly briefing. Minutes later, they splash off into the cave with guides front and rear.
Another local hovers with a digital camera in a housing, and accompanies the group. He will present the visitors with images of their adventure when they return to the sunshine 15 minutes or so later.
Snorkelling is now a popular activity at other places, such as Gran Cenote and Dos Ojos Cenotes, both of which are also magnets for those wishing to make their first scuba dive in a cave.
We had dived these two sites earlier in the week, and they bore witness to the significant improvement in safety standards in the area over the past seven years. Hundreds of people a day now seem to visit the Dos Ojos system - the Two Eyes - and the artificially illuminated, somewhat secretive Bat Cave site, which lies at the foot of
a short length of rigid steel ladder between these eyes.
You can now reach Dos Ojos comfortably by car, but one of the larger operators in the area, Hidden Worlds, will transport day visitors using its more adventurous junglemobiles.
This is an experience never to be forgotten, as you bump and rumble over rough track while clinging to a machine thats more Mad Max than road-going.
Notice-boards and careful briefings above ground, and well-laid lines and appropriate ratios under water, ensure that a cavern tour is as enjoyable and stress-free as possible. With large, warm, clearwater tunnels and shallow depths, cavern-diving has come of age here.
You swim between well-sculpted rock formations, giant draperies of flowstone, with your light piercing the darkness 20m or more metres ahead.
In a 3-5mm wetsuit, a dive of an hour is possible without hood or gloves, and you get out of the water beaming!
Cenote dives really are accessible to all. Basic scuba divers, at say PADI Advanced or equivalent, can dive up to 11 superb cavern sites without specific cave or overhead-environment training.
These sites include Grand Cenote, Temple of Doom (Calavera), Dos Ojos, Tajmahal, Chac Mool, Ponderosa (El Eden), Carwash, Casa Cenote, Bat Cave, Angelita and the Pit.
Divers are taken on a tour with at least one guide per four students. They must comply with normal cavern safety limitations - maximum penetration from the surface 60m and depth 30m; daylight visible at all times; use of guidelines; no passing of restrictions
and no decompression.
Those who are Intro to Cave trained or above can rent gear and tanks and dive on their own, but unless they have considerable knowledge of the cave to be visited, in particular the layout of lines, the sheer complexity of the sites can prove daunting.
Diving is a recreational activity, and if the number of smiling faces we saw is anything to go by, Mexico has the key.
Yes, there are mosquitoes and other little nasties. We took anti-malarial prophylactics, though medical advice suggests that in this area such measures are not essential. I found a scorpion on my hotel-room floor one morning, a little creature that must have crept into my bag when I was deep in the jungle.
In cave entrances you find bees nests and, in one cave I photographed a green-headed tree snake hanging from the ceiling. Whether it was resting or trying to catch bats or swifts I dont know, but anyone interested in the natural environment will marvel at these sites.
On several days we saw badger-sized coatis in the jungle. You hear these clumsy creatures long before you see them, and have to ask why they even try to climb trees. Seeing them fall from the branches is entertaining!

ON THE LONG HIGHWAY SOUTH from Cancun, you notice how flat the landscape is, with jungle stretching inland as far as you can see.
But just a few metres beneath the surface lies another world, a labyrinthine network of tunnels. Little wonder that the ancient Mayans saw the cenotes as portals to another realm, another life.
And while the jungle is a haven for wildlife, this subterranean world is also a repository of natural wonders.
In the saltwater section of the Temple of Doom cenote, I see for the first time a small crawling Remipedia, akin to a large white centipede and a species discovered only in 1980.
On our final dive of the trip, Helen and I spent 136 minutes on a single dive in Nohoch Nai Chich. Our depth was just 9.5m. We traversed kilometres of fabulous cave tunnel, walls and ceilings draped with stalactites and stalagmites.
It was like diving in and around an elaborate wedding cake, or through some ancient cathedral where a million candles had been left to drip onto whatever lay below. We saw a rare white cave fish of the Lucifuga family, and a number of similarly blind cave-dwelling isopods, Creaseriella anops.
There were quite a few people at sites such as Dos Ojos and Gran Cenote, but elsewhere, we never saw a soul. Under water, once beyond the entrance cavern zone, we met other cave-divers only on a couple of occasions.
Idyllic as the area appears, Yucatans cenotes are under threat. The town of Tulum has 18,000 residents, but environmentalists are concerned about plans to transform it into a city of a million people over the next 30 years.
Miles of jungle would be cleared to create hotels and golf courses and an international airport.
Passing through the magnificent corridors and chambers of Nohoch Nai Chich, we savoured some of the clearest waters in the world, yet even our presence was generating a problem for the environment.
Outside the cave, a small shoal of Mexican tetras (Astyanax mexicanus)had welcomed our arrival; these are members of the piranha family and had their own agenda that day.
Throughout the dive these small fish accompanied us, using our lights to forage for food. It was fascinating to see how these serious cave predators dashed in a panic towards Helen, 20m away, when I turned my lights off and swam in darkness for a while.
I am often asked to name my favourite cave-diving destination. For sheer enjoyment and elemental beauty, I have to say the cenotes of the Yucatan peninsula, the most intricate network of flooded tunnels you will ever find.
Lets hope the powers that be heed the voice of the environmentalists.

The Yucatan cenotes rank among the longest caves in the world. Jim Coke was one of the first people to dive them, and is probably the leading authority on cave data and map compilation.
He says there are now well over 432 miles of underwater cave passages in the area.
Last year German diver Robbie Schmittner and Englishman Steve Bogaerts hit the headlines when they established the longest underwater cave in the world.
This is the incredible Sac Aktun complex of tunnels, which stretches 96.6 miles from Gran Cenote in the west to Nohoch Nai Chich/Dos Ojos, some six miles east as the crow flies. The network has any number of entrances and many miles still to be explored.
The record was snatched away a month or so later when it was realised that the major Ox Bel Ha complex, further west, was already longer at 102.1 miles.
Unbowed, Schmittner and Bogaerts are now attempting to connect Sac Aktun to Dos Ojos, which would put the overall length 31 miles or so ahead of Ox Bel Ha.
Their accurate surveys and precisely timed dives have resulted in an audible (knocking on the walls) connection between the two caves, but as yet a diveable link eludes the explorers.

Following a guideline in the Nohoch Nai Chich complex.
A group of snorkellers explore one of the many caverns.
On a platform in Dos Ojos.
Divers in Gran Cenote.
Tetras at Nohoch Nai Chich use divers lights to hunt for prey.
Martyn Farr

GETTING THERE: Flights to Cancun, usually via Miami. No visa is required. Tulum is about two hours drive south on the main coast highway. Operators can arrange transfers but you get more flexibility by hiring a car at the airport.
DIVING & ACCOMMODATION: Martyn Farr found Xibalba Dive Centre at Tulum very helpful and professional. Owner Robbie Schmittner can arrange accommodation as well as transfers to and from Cancun,
WHEN TO GO: The quiet period is from October to the start of December. After the New Year peak it quietens down again and conditions are excellent to April and beyond.
LANGUAGE: Spanish, but English common.
MONEY: Mexican Peso, but US dollars widely accepted.
PRICES: Flights from around 370. Transfers costs around US $90 return, Hotels in town are $45-90 per night/room with breakfast, www. Beach cabañas cost from $90-130, www., Expect to pay $65-75 for a single cenote dive, including equipment, entrance fee and dive-centre services. A two-tank dive costs $100-120.