Cozumel... even the name sounds like being dipped in caramel. I flip backwards over the side of the small white speedboat into sea that is a warm and welcoming blue. I slide beneath the surface as Pedro Pablo, my dive guide, and a handful of other divers come spiralling past, and I join the loose shoal as it heads downwards.

We are diving the Devil's Throat, a suitably evocative Mexican name for the twisting cavern that skewers the coral and exits at 36m over a sheer drop-off. It is considered an advanced dive, and the Blue Bubble Dive Centre scrutinised my qualifications and observed me on a check-out dive before allowing me along.

I am admiring the spectacular trails of silver bubbles as the group enters - one diver at a time - a short cavern at the top of the reef. There are several on the way to Devil's Throat.

Navigating them requires good buoyancy control and gentle finning well away from the sand. The key is not to rush or flap. Any sign of panic or ineptitude is likely to result in you being kept out of the main attraction.

My small torch shows the inside of the cavern to be covered with colourful sponges. A large crayfish lurks at the back of a crevice in the ceiling. The thrill of the cavern is eclipsed by the glorious sensation as you exit into bright blue. The coral here grows in big columns, like crazy, stunted trees. A maze of paths and arches, formed where the columns have joined and grown together, is used as cover by a wealth of reef fish, and patrolled by barracuda and turtles. As the downward slope steepens, the clumps make way for the solid coral wall of a huge drop-off.

We head deeper, narcosis and heightening anticipation making the caverns seem narrower and more mysterious. I gingerly make my way around a turn and the passage opens into a large chamber. Pedro is waiting inside with his torch, solemnly illuminating the crucifix-shaped growth of coral that inspired the cavern's name.

We exit at 32m over a stunning wall and run straight into the hypnotic deep blue of the open sea, dotted with thousands of feeding fish. Fabulous.

Pedro is beside me to ask about my air. More than 100 bar, and a final swim-through awaits, but we mustn't dawdle. I experience a warm buzz of excitement as the darkness swallows me. The abrupt end of the passage leaves me staring down the intoxicatingly sheer wall of the drop-off. Major wow.

We weave our way upwards through the coral forest back to about 14m, where the coral forms a plateau. The joy of diving in Cozumel is the constant, lazy drift which allows you to lie motionless in the water, as if on a sofa, and simply watch the scenery and marine life go by. A lone spotted eagle ray swoops into view and heads back out into the depths, causing several of the group to fall off their imaginary sofas and perform excited pirouettes.

The group is still gripped by the thrill of the dive as we climb back onto the boat, which, magically, appears to have followed our progress from the surface. People who had never exchanged a word beforehand are chatting away like old mates about the dive. As I shrug the BC from my shoulders I experience a sweet, luxurious contentment. Cozumel... it really is like being dipped in caramel.

As most of the hotels are on the waterfront, the boats will pick you up from the jetty behind yours and drop you back. Marvellous. I'm staying at the Barracuda, about 15 minutes' walk from the town centre and a genuine divers' hang-out. You can walk through the lobby in full wetsuit carrying your soggy dive gear, and the receptionist will just smile and wish you a nice day.

If you dive in the morning, you get two dives before lunchtime. If you're particularly keen, you can fit in another two in the afternoon, and total diveheads can come back for a night dive.

There are more than 100 dive centres on the island of Cozumel. I avoided the cattle-truck approach of the larger high-street centres and plumped for Blue Bubble because it specialises in small groups on nippy boats and is happy to take you to the farther and more interesting dive sites.

Flights arrive in Cancun - the Costa del Sol of Mexico's Caribbean coast - but most divers hire a car or taxi and take the region's one main road heading south, to Playa del Carmen. This funky, cosmopolitan town has Mexican, Californian and Mayan influences. It's the Camden Town of Yucatan.
From here you can take a short ferry ride to Cozumel, or drive further south to Akumal, an eco-tourism centre with a great dive shop that serves cave-divers, and beaches where turtles lay their eggs. Further south you meet the picturesque Mayan ruins overlooking the turquoise sea at Tulum.

The dense jungle and cenote entrances on the inland side of the road contrast with the gateways into the slick tourist resorts on the coastal side, such as Xel-Ha and Xcaret. These offer a snorkeller's paradise and the opportunity to swim with dolphins.

I'm here for the cenotes - freshwater sinkholes that lead into a vast network of underground caves with astonishingly clear viz and gobsmackingly strange and beautiful interiors.
Kitting up to dive in the jungle is a surreal experience. I swipe at mosquitoes while pulling on my wetsuit, and heave the twin tanks onto my back. If you stick to the cavern zone - within sight of the entrance - your guide will usually let you use normal scuba gear, but for venturing further in, more specialist equipment and a cave-diving qualification are compulsory.

A short yomp through the jungle and I fall into the water as elegantly as the slippery rocks allow. Feisty shoals of sticklebacks dart around me in all directions, flashing silver through the green pond weed. The dark overhang of the cave entrance beckons.

Attaching my line to a protruding branch, I reel off into the darkness, following my torch beam and searching for the 'gold line' - a yellow guideline laid by previous divers to help others explore the cave system safely.
Glancing back at the window of bright daylight framed by the cave entrance, the water is so clear that I can make out the trees of the jungle and the blue sky beyond. It looks exactly how I imagined the gateway to heaven as a child. I turn towards the darkness, saying a quick prayer to the Mayan cave god as I start my exploration.

inside the Gran Cenote in Yucatan

Louise with dive guide Pedro Pablo at Devil's Throat

coral outcrop


GETTING THERE: Continental and American Airlines fly via Houston or Miami.
DIVING AND ACCOMMODATION: Louise Trewavas booked the Barracuda Hotel in advance (e-mail barracud@ cozumel. com.mx), flew in and checked out several dive centres in person before settling on Blue Bubble (www.bluebubble.com). Many centres are online at www.playadelcarmen.com and www. cozumelonline.com. For cenote diving see www.cozumel-diving. net/yucatech/ and www.akumaldiveadventures.com
WHEN TO GO: Any time of year, though some prefer to avoid October, which is traditionally hurricane season.
DIVING SUITABLE FOR: Everyone. Great easy dives for beginners as well as a range of more challenging experiences for the hardcore diver.
FOR NON-DIVERS: Fantastic swimming and snorkelling, great beaches and bars, jungle walks, Mayan ruins and stacks of fabulous shopping opportunities. COST: Kuoni Travel (01306 747006) and Crusader Travel (020 8744 0474) offer flight and accommodation packages from£499 for 7 days, not including diving.
FURTHER INFORMATION: Mexico Tourism Board 020 7265 0704, www.visitmexico.com