As the anchor slid into the Andaman Sea, a hush of anticipation descended on the boat. Here we were, a dozen or so tourists, dwarfed by the limestone karsts that punctuate the seascape between Phuket Island and Krabi on Thailand's southern shore, looking forward to our adventure.

I for one did not really know what to expect as the crew unloaded the bright yellow kayaks. Will I have to paddle? Will I be able to find my way in the dark caves?

My questions were soon answered as the head guide gave the briefing. Once we had put on our life vests, he explained, we would pair-up and join one of the guides waiting in their kayaks which by now had spread out around the boat like ducklings around their mother.

Stepping gingerly onto my allotted kayak, I grinned nervously at my guide who beamed a typical Thai smile. If you know Thailand you will know what I mean: the type of smile that can be seen from space. Our guide, 'Nit' was his name, paddled us effortlessly away from the mother ship and closer to the, by now, imposing cliffs of the island, the first of many we would be getting to know intimately that day.

This excursion was pioneered by an American John 'Caveman' Gray. It was in 1989 that John discovered the 'hongs' of Phang Nga Bay in Thailand: the Thai word 'hong' means room and it refers to the open area inside these limestone islands. If you can imagine that some of these islands in Phang Nga Bay are shaped like donuts, the type with a hole in the middle. John discovered that at low tide it is possible to follow a network of caves through the limestone rock into an inner world.

In that same year John set-up his company, Sea Canoe; and so successful was his concept that he spawned many copycat companies including more than a few Sea Canoes. To avoid confusion John has re-named his company John Gray's Sea Canoe.

'Nit' explained in his very good English that at certain points on our journey through the caves we would have to lie flat on our backs because of the low ceilings. You can imagine at this point I was beginning to question the sanity of my decision to drag myself out of bed early that morning simply to put myself in mortal danger. At the same time I was trying to work out which way the tide was running. If these caves were only accessible at low tide, how much time did we have to get in and out before the tide turned?

"And when you get inside", Nit's voice interrupted my thoughts, "to protect the environment, there is no talking and no smoking". Excellent, now I remembered why I was risking life and limb. It was this idea of visiting a pristine environment that had first attracted me.

The caves were becoming narrower now as Nit manoeuvred our kayak through the dark. "Lie down" he whispered as his torchlight captured a low hanging rock. The ceiling of the cave was getting closer to my face and I had to turn my head to prevent my nose scraping along the sharp limestone. This was actually getting exciting although I wasn't sure how long I could cope with this claustrophobic feeling that was starting to creep over me.

But within minutes the cave started to open up again and a hint of daylight caressed the cavern wall.

I squinted as we emerged into the bright sunlight, into a land that time forgot. Trees were clinging to the steep scrub-covered walls of the 'hong' and reaching out for the brilliant blue sky above. A sea eagle swooped down low to see who these interlopers were. And all was peace and tranquillity.

The family of kayaks paddled slowly through the hong, the occupants in awe of this magical, secret place in southern Thailand. The only sounds were the lapping of water, the call of the cicadas, the clicking of cameras and whispers of excitement.

All too soon it was time to head back. John Gray's professional guides were well aware of the movement of the tides and making sure we were through those caves in good time.

The excitement only increased with each 'hong' we explored and the memories of that day will never leave me.

This Sea Canoe day excursion is available from Phuket, Thailand: it is a relatively expensive one but for me it was worth it.