Those who do take to the sport find a beautiful subterranean world - and perhaps find out a little something about themselves too.

It is a hobby pursued worldwide with growing interest. It is also a pastime in which participants constantly look for more and more challenging holes to explore.

So why do people do it?

When I was about 17-years-old, I and friends crawled through a smelly crevice thinking it would be a good idea to see what was at the other end.

Ten minutes later we ran out of socks to burn for light and spent the next 30 minutes struggling backwards on all fours.

Years later we all decided that if we were going to spend our time squeezing though slimy wet spaces, then we had better do it properly.

Caving is perhaps the most extraordinary outward bound activity I have ever had the pleasure - and the pain - of pursuing.

Cave diving - the most risky pursuit on the planet?

It is clearly not for everybody. While those of us who do it kid ourselves that it is not as risky as rock climbing, it is arguably more dangerous.

If you break your leg down there, it is going to be an extremely difficult job getting you out.

But those who do take to the sport find a beautiful subterranean world - and perhaps find out a little something about themselves too.

Amazing world

Here are the good bits: I have slid down a smooth black helter-skelter of a tunnel, millions of fossils illuminated by my headlamp as I descended.

In New Zealand, I have splashed through cave water, prompting microscopic glowing insects to light up the cave ceiling like a second zodiac.

I have seen the most delicate stalactites which have taken hundreds of thousands of years to form - yet would crumble if touched by the human hand.

Physical challenge

But for many of us, the physical challenge, remains at the heart of the endeavour.

Cave openings are often difficult to reach

Sports caving is generally well organised. Local groups look after caves and carefully restrict or monitor their use for both conservation and safety reasons.

The equipment has to be as rigorously checked as that for mountaineering. The basic clothing comes in two parts:

• A body fleece for warmth with a tough and stiff outer layer for protection
• Wetsocks to keep the feet warm and rubber Wellington boots for grip and protection.

Then there are the absolutely essentials: The helmet, the headlamp, safety harnesses (for novices) rope, ladders and climbing gear.

Getting into a cave can prove the most difficult part of the endeavour. Many cave entrances are small or involve vertical drops.

One cave entrance I remember was so narrow that you could only get down by completely relaxing and, literally, trusting your body to fall to the bottom

One cave entrance I remember was so narrow that you could only get down by completely relaxing and, literally, trusting your body to fall to the bottom.

Once inside, sports cavers follow the natural water courses and use maps prepared by others to navigate up and down, left and right.

There are pitches (the drops) where you must either climb, abseil or use a lightweight rolled-up steel ladder to navigate.

There are wonderful caverns where you sit and chew Fishermen's Friends for warmth.

Into the squeeze

Then there are the squeezes - and turn away now if you are of a nervous disposition.

As his breathing rate rose, he expanded and plugged the gap. It took him 10 minutes to relax enough to be able to slide through

A classic squeeze would be a tiny tunnel between two rock faces. It is 12 inches high, not a lot wider and 20 metres long. Water is about an inch deep along it's length. And so down on your belly you go.

Your cheek presses into mud that' has been lying there since the Jurassic period and you can only just about flex your feet enough to inch your way along. Someone cracks a joke and you do not know whether to laugh or cry.

Squeezes can also prove problematic for the most experienced. I have seen a friend (far more experienced than I will ever be) get stuck at a point appropriately called the "letterbox" just five minutes into a cave.

As his breathing rate rose, he expanded and plugged the gap. It took him 10 minutes to relax enough to be able to slide through.

Safety and dangers

All the cavers that I have met have been professionally scrupulous with their safety planning and would never force any novice to do something that they did not feel comfortable about.

There are two caving activities out of bounds for most enthusiasts: Cave diving (the use of scuba equipment) remains arguably the most risky pursuit on the planet.

Secondly, some cavers spend their days mapping and finding new tunnels - sometimes with the help of licensed explosives.

But perhaps the most dangerous times for most enthusiasts come when experienced cavers have too many novices with them.

If a beginner psychologically "refuses" at a difficult point, they need to be taken out quickly to prevent them becoming a danger to themselves and the group

Each novice creates delays - I know that I did. Someone will inevitably panic. Someone may not be strong enough to complete a climb. Everyone gets colder, the risk of accidents increases.

If a beginner psychologically "refuses" at a difficult point, they need to be taken out quickly to prevent them becoming a danger to themselves and the group.

There are the geological dangers too - the potential for falling or dislodging a rock and, of course, water.

Torrent

The ground can only hold so much rainwater before it dumps it further underground - that is how caves are formed.

The best way to visualise how rainwater enters a cave is to think of how a trickle at the top of a funnel can be a torrent at the bottom. You, of course, are standing at the bottom.

Undoubtedly, the worst situation would be to find yourself trapped in a higher cavern, knowing that the only way out would be to dive through a u-bend of rising water.

But for many people, it is also in these situations that you grow as a person, knowing that you have mastered a fear

Thanks to the experience of those who have led every expedition I have done, I have never faced that decision.

But for many people, it is also in these situations that you grow as a person, knowing that you have mastered a fear.

About two years ago my group leader noticed that the water was clearly beginning to rise around our ankles.

Freezing

We began climbing up a final narrow vertical chimney as hundreds of gallons of water crashed down onto our heads, making breathing and seeing particularly difficult.

The rocks were jagged and stabbed at our bodies as we climbed. I accidentally stood on a colleague's head, someone stamped on one of my fingers.

It took us 10 minutes to complete the climb to the open, all of it through freezing water.

Drenched and covered in cave mud, I grinned at my friends as we greeted the grey rain clouds overhead. That night's Indian takeaway tasted oh so good.