WE WANDER DOWN the overgrown path beneath a dense canopy of tropical trees. The air is static, the mosquitoes are relentless and the heat is overwhelming.
A few more steps and we find ourselves on a beautiful wooden deck at the mouth of a wide yawning cave. This is the entrance to Dos Ojos, one of the most famous cenotes in Mexico and our first serious foray into the realm of cave-diving.
We kit up and drop into some delightfully chilly fresh water, then descend to 5m. Our dive-leader points at the guide-rope, then indicates that we should follow him into a black hole.
We can only just see the rope as it catches reflections from the daylight behind until, in true theatrical form, the guide switches on his torch, and an utterly breathtaking vista of submerged limestone rock formations is revealed.
We explore the twists and turns of this amazing sunken network of tunnels and caves until we exit astounded, excited and desperate to repeat what has been a totally surreal experience.

First steps into the dark
Naturally, we had had concerns. We had only ever finned into caverns from which we could see the way out, sunlight beaming inwards from the exit point as well as the entry.
As for most people, diving was all about floating about in a relaxed environment, pretty fish swimming by, the occasional encounter with a creature far bigger than us, and generally passing time without a care in the world.
Cave-diving is the antithesis of all that. It’s dark, it’s spooky, mysterious and intriguing and frankly, it can be dangerous if you don’t take the right precautions.
Yet the interior of inky caves is an environment rarely seen, including creatures you only ever read about or see in the movies. However, with some planning and a little extra care there is no reason why even novice divers can’t take their first steps into this fascinating realm. Here are six golden rules:

• Make your first cave-dives in a well-organised location with a fully trained guide.
• Listen carefully to the briefing so that you can visualise where you’re going
• Take three light sources, to be sure, to be sure…
• Keep an eye on your air or nitrox gauge and commence your exit as soon as you are one-third of the way down your tank, to be really sure.
• Be especially careful with your buoyancy and your fins, so that you don’t kick up silt.
• If you enjoy it, consider taking a cave-diving course.

Most caves that divers see were formed in limestone rock in the far-distant past.
Over the millennia, rivers and streams or volcanic activity created holes and channels, while rainwater on the land above percolated down through the limestone, eroding the holes further and eventually creating vast cave systems.
Inside, stalactites and stalagmites grew into magnificent formations with glittering surfaces. When the last Ice Age kicked in and the oceans levels rose, many of these caves were flooded and their entrances often concealed – although not to adventurous divers.
There are cave-dives all around the world – some are close to home, some are really meant only for the pros and technical divers, but there are just as many that are perfectly appropriate for sport-divers.

The Mediterranean
Close to home, easy to reach and well-known for its many caverns and caves, the Med is a gateway to learning how to cave-dive, and you can easily arrange to take a course in shallow waters.
Malta is well known for its Blue Hole, which Jacques Cousteau once praised. Billinghurst Cave, at 70m long, is the longest in the country.
Less well-known as a diving destination but surprisingly good for limestone caves are the idyllic Greek islands. The diving industry was highly restricted here until recently but there are now dive-centres on lots of islands and more than 8000 natural caves, including many that are under water.
There are dive-operators on bigger islands such as Crete and Santorini, but one place that is a microcosm of diving styles is the channel between the island of Paros and its little sister Antiparos.
This smaller island is famous for a massive mountainside cave system at Agios Ioannis. The entrance is 350m above sea level, but once inside you can walk almost all the way back down (and then all the way back up the steps to the top!).
The chambers are thick with beautiful stalactites and stalagmites, as well as ancient inscriptions said to date back as far as Alexander the Great.
After exploring this marvel, you can dive the Blue Dome beneath nearby Paderonisi Island. This cave is small but the geological formations reflect those at Agios Ioannis. The entry is via a chimney at the base of the tiny island and rises from 20 to about 5m. You can surface inside to see the stalagmites and stalactites hanging above and illuminated by an eerie blue light coming from below.

The Yucatan Peninsula, on the edge of the Caribbean Sea, is ideal for budding cave-divers. Reefs around the lively island of Cozumel are riddled with swim-throughs and tunnels that will give you a feel for cave-diving.
Palancar Caves is a maze of spires and buttresses, gullies and canyons. Although there is often a roof over your head, there is always light and an exit point in sight.
Next, and most exciting, is the Devil’s Throat at the southern tip of the island. This dive starts in a cavern at 26m, which turns into a descending tunnel that exits on the reef wall. From there another entrance leads to a second tunnel that narrows significantly.
This slender chimney is the Devil’s Throat. It leads to an exit on the reef wall at 37m, or you can take a side-turning to the Cathedral, a vast cavern lit by sunrays filtering through fissures in the reef above.
With a few Cozumel dives under your belt, it’s time to head over to the mainland and the ancient cenotes. Probably the most mysterious and historic of any caves you will dive, the cenotes were vital to the ancient Maya. They believed that underworld gods lived in their depths, and people were sacrificed to appease them.
A vast network of subterranean rivers, underground caverns and sinkholes (cenotes) extends miles inland from the coast. Those closest to it are flooded with sea water, while ones further inland are filled with fresh water. They are all full of stalactites and stalagmites, rocks and the roots of jungle foliage.
Dos Ojos is the one most often used to introduce divers to this fascinating realm. You are always accompanied, as there are many unexplored passages with “Warning – Keep Out” signs, but guide-ropes keep you safely on the route.
Even in the more complicated cenotes, conditions are not difficult.
One of the most fascinating aspects occurs when you pass through a halocline – this is a phenomenon created by the meeting of salt and fresh water. The fresh water sits over the salt and forms a bizarre, mirror-like layer.
Taj Mahal cenote remains far more natural, as it is part of a river system that runs in a straight line away from the opening cavern. Divers swim past a series of quirkily named limestone features (Close Encounter, Cenote Sugarbowl, the Candlestick) before passing beneath a few smaller cenotes.
You can ascend into some to see the jungle before dropping down and finning below invisible fissures that allow rays of light to illuminate your route.

Next up in the cave stakes and perhaps one of the world’s most exciting and varied dive destinations has to be the islands of Palau. This small country has a huge variety of natural environments and lots of enormous caves with intense blue-hole openings that avoid any feeling of being enclosed in a tight space.
The dive named Blue Holes is where you descend through any one of four holes in the top of the reef into a single huge cavern with crystal visibility.
Light falls through the chimneys above and creates sparkling blue reflections on the sand far below.
There are several similar dives around the island but Chandelier Cave is perhaps the pinnacle. The entrance to this hidden cave system is in a lovely cove just across from the port. You swim inside via a gap just a few metres below the surface and about 3-4m wide, but as soon as you go in, it opens to a gaping space.
There are stalactites at the beginning, but these are rounded like columns, perhaps reshaped by the movement of sea water. Heading further in there are four more chambers inside, and it gets progressively darker the further back you go.
It’s not possible to reach the last cave, which is actually above the waterline and requires squeezing through a muddy tunnel. However, you can ascend inside the other chambers to where an air-gap allows fresh water to percolate through from the land above, which creates spectacular sculptures.
The formations in these are breathtaking, as all the surfaces glitter with absorbed minerals and appear as pleats of fabric or pencil-thin icicles. It’s all incredibly pretty.

For those looking for some truly unusual cave-dives that are still within sport-diving limits, Australia’s Christmas Island is close to being unique.
The island is unique anyway. It’s known as “the other Galapagos”, because outside of those islands there are more indigenous species here than anywhere else on the planet. But back to caves and diving…
Christmas is a limestone rock that sits over harder basalt. The location in open ocean means that waves and wind have eroded the limestone, and much of the coast is craggy, steep and inaccessible.
You can explore several land-base caves, but beneath the waterline and from the shallow fringing reefs there are even more.
At Coconut Point a small sand gully under the cliff leads into a huge cavern with openings to the cliff-face on two sides. The light inside has an eerie glow and takes you to the rear, where you crab-crawl through the surge over beautifully rounded rocks until the light fades.
If you turn on your torch, you spy an incredible number of huge lobsters nestled into virtually every crevice.
For almost everyone who likes to explore caves and has been to Christmas, Thundercliff is the high point. A dive, a swim and a walk, this is probably one of the most unusual underwater experiences you will ever have.
Entering over a shallow section of reef, you swim down into a sandy channel, under an overhang and into a wide-mouthed cave. A short fin takes you past a rock that juts almost to the surface and has hundreds of silvery sweepers swarming around it.
The trek continues by passing through a narrow tunnel until you reach a completely dark cavern where you surface inside to admire the stalactites overhead.
Descending again, you swim into a second, tighter passage that leads to a much bigger cave. Surfacing a second time, you find an enormous space with soaring proportions. Every surface is decorated by impressive limestone structures.
And if all that wasn’t enough, you then swim up to a beach and exit onto the rocks before dekitting for a walk through the cave system. A little way back is a small pool of brackish water, and inside that is a rare red shrimp (as yet unnamed) that is attracted to torch beams.

• First cave-diver: Jacques-Yves Cousteau is regarded as the world's first scuba cave-diver, although many divers penetrated caves before him using surface-supplied breathing apparatus with hoses and compressors.

• First British cave-dives: Swildon’s Hole, an upstream feeder for Wookey Hole in Somerset, was first explored in 1935, but full exploration of this extensive cave-system was delayed by WW2.

• Deepest cave-dive: Portuguese diver Nuno Gomes holds the official current Guinness World Record for descending to 282.6m in Boesmansgat cave in South Africa.

• Most famous cave-dive: Belize’s Blue Hole is a perfectly circular pool that drops to 146m deep. The walls are sheer to around 33m, where you see a few stalactite formations dripping from an overhang.

• Most popular caves: More divers train to be cave-divers in Northern Florida than anywhere else, attracted by the variety and range of sites in crystal-clear water and the chance to meet manatees.

• Longest surveyed underwater cave system: The Yucatan’s Sac Actun was in front with a measure of 230.8km until Ox Bel Haat pipped it at 256.7km. However, the Dos Ojos system was found to connect to Sac Actun and together at 319km is now the longest cave in Mexico, and the second longest worldwide.