“BY THE RULE OF THUMB, A MOUNTAINEER IS AN ADVENTURER” said Roger, a caving Instructor. I knew what he was getting at, but I wasn’t the least interested in going in to dark and dank underground places. I like the openness of the wide open valleys, beautiful vistas, fresh air and independence. Caving to me is claustrophobic but when you are upgraded to an adventurer, it is hard to pull out.

Roger, a fit-looking forty-something was going to be our instructor and leader the next day. We chatted for an hour or so during which he tried to explain the similarities between climbing and caving. “Oh, you climb up and then climb down, while we climb down and eventually have to climb up. It is no different than what you do” he said trying to make it look simple and easy. I couldn't decide whether to feel encouraged or not.

MENDIP UNDERGROUND. Unknown to him, I had been reading Mendip Underground, a caver's guide written by local experts Tony Jarrat and Dave Irwin. There was no mistaking what some of the technical terms foreshadowed: a low grovel, a muddy wallow, a wet flat-out crawl and a. desperate squeeze did not need translation.

The names of various features of the Mendip caving systems seemed similarly mixed in portent. The Ruby and Crystal Chambers, Princess Grotto and Harem Passage sounded appealing. But the Vengeance Passage, Agony Crawl and Sludge Pit Hole didn’t sound too good.

GOING DOWN. As Roger unlocked the door to the entrance to GB Cave the next morning, I started to think of Something Nasty In The Attic and Abandon Hope. Dressed in waterproof suits, helmets and lamps, I gazed down a squarish hole into darkness. Holding myself, I followed Roger down the hole, picking my steps on the metal ladder by the light of my lamp. It was dark and damp.

The cave was 3ft wide and straightened up with chambers out on each side and overhead. I could hear the scrape and shuffle of Jack bringing up the rear, a reassuring sound. We came to a place where the passage roof dropped sharply and the sides, narrowed in. My helmet lamp showed a short tunnel through the rock, roughly two feet square and maybe 5ft long.

The soles of Roger’s rubber boots gleamed momentarily in the opening, and then he was through with an athletic wriggle. I got down on my belly, shoved my arms out ahead, and wormed forward. It was impossible to squeeze through. The rock around me felt as if it was in-contact with every part of my body except my face, and that was laid ear-down in a puddle of mud.

“Turn on your side," came Roger's calm advice, "and just pull yourself forward." I did as he said but nothing happened. I was stuck. “Go back and try again” he said calmly again and again it was the same. After learning a few tricks lying down on the muddy floor, I slowly slithered out on the far side, as greasy as a new-born baby. Later, flicking through descriptions of desperate squeezes and right-angle corkscrews in Mendip Underground, I found that the little funnel I had passed through did not even rate a mention.

Then came a couple of 15ft descents down slippery looking walls of rock. That would be easy, I thought. With some experience in climbing and guidance from Roger about the cracks and hand-holds, I found I could spider my way down the vertical rock faces at the cost of a barked knuckle or two. It wasn't until the following morning that muscles I never even knew I possessed, began to complain about being woken from their 50-year slumber.

As we walked, crawled and crouched our way south, a couple of hundred feet below the grazing fields of Mendip, my lamp picked out gems of underground architecture: smooth curtains of cream-coloured flowstone coating the walls, tiny pale needles of stalactites dripping from the roof, helectite in hunches like coral or frosted cauliflower heads. "Calcite," said Roger. "You never get tired of the different ways it forms. Always different, always beautiful."

BEAUTY BELOW GROUND LEVEL. Turning around a corner, I found myself suddenly gobsmacked. What I saw was a breathtaking sight. GB's main chamber opened up in a vast cavern, nearly 100ft from floor to roof. A rock bridge sprang out across a ravine. Long stalactites hung among wavy curtains of calcite and haphazardly sinuating worms of helectite. Nearer at hand were smooth, round knobs of stalagmites, hobbling the chamber floor.

When caving first took off as a serious science-cum-sport in the 19th century, cavers would snap off calcite formations and knock down stalactites for their rock gardens. Finders could be keepers in that carelessly innocent climate of opinion. Nowadays the emphasis is on responsible caving: enjoy it, and leave it for others to enjoy. "Don't touch the stalagmites," Roger warned when he saw me stretch out a hand to the irresistibly rounded shapes. "We try not to spoil the decorations."

I knew well enough the kind of pleasure I had got out of my few hours underground: the sense of achievement in overcoming squeezes and rock climbs, the beauty of the calcite formations twinkling in lamplight; the feeling of being somewhere truly "other", yet part of a familiar landscape.

I call myself an adventurer but that is an old term that has long lost its true meaning. The real adventures are the cavers and what they do is real exploration.