You will bounce through the interior over 7 miles of very rough road, ploughing through a creek that can be quite deep (depending on how much rain there has been - the bridge was washed away during Tropical Storm Arthur in 2007, and they're still working on its replacement). You will pass orange and grapefruit groves, and other crops, here and there, in ground wrested from some awesome scenery: untamed, thickly wooded hills framing what feels like a valley to the back of beyond.

As long as the tour vehicle doesn't break down under the strain, you will stop only at a farm which is doubling as an archaeological station, where some money changes hands. A little further on the road stops, and from now on you're on foot. For those needing a toilet break, find your own spot, ladies to the left, men to the right. You will be issued with your helmet, your lunch and some additional water, and then it's follow your guide through the jungle time.

To get to the mouth of the cave you walk for about 40 minutes (it's a fairly easy trek, fording a shallow river three times to a fairly crude campsite (known as the Xibalba Hilton (Xibalba - pronounced Shibalba - is the Mayan word for the Underworld). There you hang up your backpack containing your lunch, strap on your helmet and your headlamp, use the facilities (Joke! - it's rainforest to the left and rainforest to the right: the great outdoors is one big toilet), hand over your camera. Leave everything else behind (both hands free and nothing in your pockets) and make your way down the slippery, root-veined slope to the mouth of the cave.

You swim in - it's about thirty feet; non-swimmers can make it around the edge clinging to the guide's backpack - and follow an underground passage through twists and turns for about half a mile, sharing the space with fruit bats, during which time you will be in water sometimes up to your knees and occasionally up to your neck.

Soon after setting off you may get to turn off your light and turn to face the entrance to see a remarkable sight: the profile in stone of a massive, unmistakably Mayan head, which it is easy to imagine is the Rain God Chac looking out towards the entrance to his world from ours. Some guides will keep your lights switched off and let you walk for a hundred meters or so in the pitch dark, just to know what that feels like when one of your key senses is switched off, and you know that you are in a tunnel under the earth, and you can hear and feel the water swirling around you...

After about 20 minutes of this wading (even a little swimming if you're under 5'4"), clambering, squeezing through and sliding down rocks, passing amazing crystalline formations, you reach an escarpment which you climb (about 20 feet) to reach a plateau leading up to a vast underground chamber of cathedral like proportions. It is easy to imagine you are at the entrance of a Hall of the Mountain Kings, or the dwelling place of a God.

It is here that the Maya made ritual sacrifices to the Rain God Chac more than a thousand years ago. This is where you take your shoes off, and follow carefully in the footsteps of the person ahead of you: for you will be threading your way through shards of ancient Mayan pottery, and the calcite encrusted remains of 13 human beings, the majority babies and small children. Your guide will hand you your camera, and create some amazing photo opportunities with his big flashlight.

Stalactites and stalagmites grow at different speeds depending on the conditions; but always, when compared to the human life-cycle, incredibly slowly: so you can only hazard a guess at the millennia it must have taken for the vast columns to have formed in that space. All around you the process is still going on: fantastic shapes writhe and twist down from the ceiling and up from the cavern floor. It's as if a cathedral grew here, with pillars, chandeliers and statuettes. When the naked flame torches of the Maya were flickering in there, and the shadows were flitting and dancing round the walls, it is easy to understand why they thought they were inside the home of a God, in the heart of the Underworld - Xibalba.

After a little more climbing, twisting and squeezing your way to the back of this space, you reach an aluminum ladder roped to the wall. This you climb to reach the end of your journey: a narrow space containing the complete skeleton of an 18 year old girl (The Crystal Maiden), now involved in the rock, legs spread, mouth wide as if frozen in that final scream more than a thousand years ago. What must her life have been like? What must her death have been like?

You come back out the way you went in: retracing your steps, putting your shoes back on, getting back into the water, re-wading that half kilometer (and now you're tired). It is with some relief and a sense of real achievement that you pass the impervious stone Chac to emerge once more into your world, where there is light and warmth, leaving him to his dark domain, adorned with the relics of those left behind for him around 950CE.

By now you will certainly be ready for your lunch, which you liberate from your backpack and consume alongside your munching fellow survivors; after which the trek back to the tour vehicle and dry clothes is a doddle, compared with what you have just been through.

It is an awesome experience that we have seen seven year olds and seventy year olds relish: but you need to be fairly confident in water, not to mind tight squeezes in small places, and to have fairly good knees for the climbing bits. Oh yes, and not to be afraid of the dark...

You will know from the above description whether it is for you or not.