The heat can become overwhelming, trudging through the jungle wearing a wetsuit, booties and an air-tank. Following my Mexican dive guide Arturo down a gravel track, I recall a story I once heard in a bar. A man dressed in full scuba gear is found dead in the middle of a forestwith no ocean around for 50 miles. How did he get there? The answer to that riddle lay in some absurd lateral thinking: a helicopter fighting a forest fire scooped up a tankful of seawater and inadvertently netted a diver in the process. It then dropped its load 50 miles inland, extinguishing both the fire and the life of the hapless diver in one fell swoop.
Simmering inside my 3mm suit, I think of a simpler solution: the diver had gone in search of a cenote - one of the natural gateways to the network of underground rivers that riddle the Yucatan peninsula - but got hopelessly lost and dropped dead from heat exhaustion. Thankfully, Arturo knows the way - and it is only a two-minute walk from where he has parked on the roadside to Calavera, a sinkhole in the limestone floor that drops into a twinkling freshwater cavern 2m below.
"We call this cenote Temple of Doom," Arturo says matter-of-factly, "or Skull Cave." My expression betrays the fact that I've never felt less like Indiana Jones. "Look over there - you see two more holes: two eyes; one mouth." I note the facial features that are allowing shafts of sunlight to penetrate the waters below. "You'll see. It's very magical," he says, and hands me two torches.
We are cavern diving, Arturo explains. The difference between cavern and cave diving is that caverns always have a source of sunlight. Anyone with an Open Water diving qualification can sign up for cavern diving, but cave diving, which takes you beyond the sunlit areas into very dark, tight spaces, requires rigorous training: we won't be straying beyond the cavern walls.
Arturo clasps his mask and respirator to his face and drops into the cavern mouth. I follow his lead, and after we both indicate we are OK, we begin our descent. The sun casts emerald rays through the three holes in the domed ceiling, creating the impression that we are suspended inside a giant jewel. Tree roots and foliage drape down and small fish nudge us inquisitively. Our torches pick out a fixed string line, which we follow down the side of a mound of sediment in the centre of the cavern into a dark corner where stalactites dangle.
Then, as if someone has placed a child's kaleidoscope in front of my eyes, the water turns oily and the colours divide into squiggles. I can no longer make out the figure of Arturo, but I have been warned of this: we have reached the halocline, the point where the freshwater meets a patch of seawater at the bottom. The two do not mix, until a diver passes through and stirs them up.
In seconds, we are through the fog and into the seawater, where full visibility returns. Arturo gives me the OK signal. I mirror him, and we continue touring the outer edge of the cavern, squeezing through narrow sections where the bottom and the walls meet 17m down, peering into tunnels where only the cave-trained dare go. The experience of floating past ancient limestone monoliths, with a faint green glow far above, is at once eerie and beautiful, and lasts for 35 surreal minutes.
Walking back to the van, Arturo enlightens me on the formation of the cave systems. "There are two theories. One is that a meteor - the one that killed off the dinosaurs - blasted holes through the limestone, and when the ice melted after the last ice age they filled with fresh water." It is true that a giant meteor did strike this peninsula 65 million years ago, at Chicxulub on the north coast - and a necklace of cenotes has been found circling that crater. But here on the east coast, Arturo is unconvinced. "I believe the other theory, that rainwaters hollowed out the limestone over thousands of years."
I am unsure which to believe. In any case, not knowing only enhances the mystery of these sinkholes. I ask Arturo if there is anywhere at sea I can witness the reverse of the halocline that I saw in the Temple of Doom - a spot where the freshwater enters the saltwater. "Head into Sian Ka'an," he tells me. "There are plenty of cenotes in the lagoons there."
The next day, I realise that driving around the Yucatan is like driving over a huge slab of Swiss cheese - the peninsula is pot-holed below and above the surface. This is especially true on the Boca Paila road from Tulum into the Sian Ka'an Biosphere Reserve, a Unesco-decreed sanctuary of more than 1 million acres of mangrove swamp, forest, reefs and empty beaches. We bump down the unsurfaced road for three kilometres and pull in at the park headquarters, where a model of a manatee stands guard outside a small museum, and a rickety watchtower overlooks the wetlands beyond. No one is about except a young, enthusiastic man called Aldo, the director of Raga Tours. For 350 pesos each (£17), we can go on a guided boat tour of the swamps and lagoons and, if we're lucky, see estuarine crocodiles and manatees feedingon the sea grass around a cenote. We sign up.
Aldo takes us down a wooden jetty to a boat driven by his colleague Julio, who whisks us into a lagoon that resembles Florida's Everglades. We motor under the Boca Paila bridge, into an estuary where the Caribbean and the freshwater mingle, and watch a baby crocodile lounging on a floating log. It does not flinch as we pass by, but it leaves us hopeful of more wildlife to come.
Julio takes us back into the lagoon to hover above a submerged cenote. A dome of freshwater is bubbling up through the brackish mix, creating a huge contact lens into which we can clearly see the deep black hole below. Although brown sea grass is billowing out of it, we feel as though we could be sucked in at any time. We pass a submerged Mayan ruin on the way to a second cenote. "This is where the manatees sometimes feed," Julio tells us. We wait in silence for the seal-like forms to appear, but after 20 minutes, alas none arrives.
Julio does not know the purpose of the Mayan ruin in the lagoon, but I wonder if it might have something to do with its proximity to the large cenote. What cannot be denied is the effect these water features had on the life and culture of the Mayan people, who have populated this peninsula since around 1500BC. In a region practically devoid of rivers, these wells were worshipped by the Mayans. Chichen Itza, the most visited and well-preserved Mayan city on the Yucatan, about three hours' drive inland from Tulum, was completely reliant on the waters of its chief cenote, the Cenote Sagrada.
The name Chichen Itza literally means "at the edge of the well of the Itza". The Mayans believed the cenote was the gateway to the "other world", or Xibalba. They threw in offerings of incense, jade and metal discs, as well as human sacrificial victims. Those who survived the 30m drop were thought to have spoken with the gods, and returned with the power of prophecy. Today, you're more likely to return with a kitsch model of Chichen Izta's pyramid for your mantlepiece.
Only in recent years have the cenotes been explored and the skeletal remains brought to light. In 2001 and 2002, three human skeletons that were more than 10,000 years old were discovered in caves, along with giant Pleistocene animals. Just 10 per cent of the Yucatan's cave systems are thought to have been explored so far, but anyone who enters them quickly learns that the Mayans were at least half right: the cenotes are indeed gateways into a magical world, but little did they know the extent of the Xibalba beneath their feet.
TRAVELLER'S GUIDE: YUCATAN
There are no direct scheduled flights between the UK and Cancun. The writer flew from Gatwick with Thomsonfly (0870 190 0737; www.thomsonfly.com) for £280 return. Fly Thomas Cook (08707 520 918; www.flythomascook.com) also flies from the UK.
Cenotes are mostly situated near roadside settlements, and are best explored from Playa del Carmen or around Tulum. The ADO bus from Cancun airport to Playa del Carmen cost 80 pesos (£3.90) for a single. The minibus from Playa del Carmen to Tulum costs 40 pesos (£1.90).
Hotel Shangri-La Caribe, Playa del Carmen (00 52 984 873 0611; www.shangrilacaribe.net). Beach-view doubles start at US$200 (£118), half board. The hotel has an excellent dive shop, Cyan Ha (00 52 984 803 2517; www.cyanha.com), offering cenote and reef diving as well as PADI Open Water tuition. Zamas, Tulum (001 415 387 9806; www.zamas.com). Beachfront double cabañas start at 840 pesos (£40), room only.
The easiest way to see the cenotes is to join a tour at Hidden Worlds (00 52 984 877 8535; www.hiddenworlds.com.mx). If you hire a car or a 4WD, you can take your pick of cenotes. The writer hired a 4WD from Executive ( www.executive.com.mx) for $50 (£29) a day. Some of the best cenotes for snorkling are Dos Ojos (250 pesos/£12 for a guided tour), Gran Cenote and Cenote Carwash (30 pesos/ £1.40 entry).
Mexico Tourism Board: 020-7488 9392; www.visitmexico.com.Reuse content