I was treading warm water in the South Pacific's sparkling Tongan islands. Five metres below me was an underwater tunnel through which I would soon swim – and I was getting last minute instructions from my guide. "Once you go through the entrance you'll have to swim into the darkness under the cliff for about 15 metres. Don't worry, you'll have plenty of time"

Time? I wasn't worried about time. I was worried about swimming blind into the darkness and an unknown fate. We were free diving, and I knew that with only one lungful of air I wouldn't be able to get back to the surface if this turned out to be the wrong cave. These, and other negative thoughts spun through my mind as I breathed deeply and prepared to submerge. My guide placed his snorkel in his mouth, nodded once, and dove beneath me with a powerful flick of his fins. With a whimper I followed suit, nervously gumming my snorkel as I stroked down toward the shadowy cave mouth.

The water was warm, but the darkness under the cliff face chilled my soul. As I stroked blindly into the tunnel my mind whispered unhelpfully "This is the wrong cave! This is the wrong cave!" I had lost sight of my guide almost as soon as we entered the cave and the further I swum into the abyss the more my doubts began to take control. After 20 seconds I knew my lungs had passed the point of turning back. My eyes started to bulge with panic and my legs pumped in double time. Then a faint silver glimmer appeared above me. At first I thought this was the onset of pre-death hallucinations, which would seal my watery fate. But as I rose I began to make out definite edges to the silvery-blue apparition. The surface! I scrambled madly over the last few meters – limbs flapping idiotically – and broke through the mirror like Alice entering Wonderland.

I found myself in a vast limestone cave with jagged sculptured walls and stalactites hanging from the roof. An unearthly turquoise light flickered around the cave. This pale light possessed the most amazing qualities after being filtered and refracted through the clear Tongan waters of the tunnel through which we had swum. It made the cratered moonscape of the cavern dance on its shifting green and blue rays – revealing mysterious depths in the hanging stalactites above me. I felt like a movie adventurer – as if I was Indiana Jones unearthing a treasure room in the sunken city of Atlantis. My jaw hung low as I gratefully gasped mouthfuls of stale cave air. I treaded water, blinking in disbelief at the exotic world that surrounded me. Even the sound of my splashing w as alien – multiplied and reflected in a crackling cacophony. I hooted in shear joy and the sound returned both amplified and distinctly warped. What a place! This was surely Aladdin's secret chamber of riches...

But the best secret of all was about to be revealed. Suddenly my ears popped and with a slurp and a hiss the cavern was flooded with thick fog. I quickly lost sight of the cave walls. The air clung to my body like a thick shroud and filled my lungs like a heavy, tasteless smoke. I held my breath in the haze and almost expected the genie of the cave to appear before me – assembled from the swirling mists – and demand in a booming voice to know why I had invaded his sanctuary. Mercifully, just as my sensory-deprived imagination began to get the better of me, my ears popped again and the fog departed as quickly as it had arrived. I found myself once again treading water in a crystalline blue cavern.

But now, because of the sharp contrast after the fog, the heavenly colour of the cave was even more intense. It seemed that my eyes had suddenly snapped into a super-human focus, full of infinite detail and fluorescent colour.

I later learned that the mists – far from being a supernatural event – were disappointingly explainable by science. The popping of my ears was due to the large pressure change caused by swells outside the cave forcing water through the underwater entrance. This compresses the air inside the cave, which in turn causes thick mist to form almost instantly. As the water in the cavern drained away so did the fog. Several of these 'clear-then-fog' cycles occurred every minute, depending on the size and direction of the swell striking the cliff face.

This enchanted place was named after Will Mariner, an early European visitor to northern Tonga's Vava'u island group. One day he was watching some local people spear fishing in front of the cliffs when he witnessed a man dive down and not reappear for several minutes. Intrigued by this, Mariner talked to the fishermen who led him through the cave's underwater entrance.

However, perhaps more interesting is the Tongan myth surrounding the cave. Legends tell of a tyrannical ruler of the island who ordered a rival family killed. The beautiful young daughter of the doomed family was saved by an admirer, who spirited her away to the secret cave to live in hiding. The admirer visited her daily, swimming into the cave with food and wooing her with exotic oils. Eventually the couple decided to flee hostilities in Tonga by sailing to Fiji and he set about gathering a seaworthy crew. As the vessel departed one of his shipmates said it would be foolish to leave for Fiji without a wife. He agreed, saying that he would have to stop and find one on the way. To the surprise of his companions, he led them to the cave, leapt into the ocean and returned minutes later with his watery bride.

So, what is Mariner's Cave?
- An aquatic treasure trove guarded by a swirling genie of the mist? Doubtful...
- The magical abode of Tongan mermaid brides? Maybe...
- How about: a natural limestone wonder waiting to be discovered by anyone with enough courage to swim into the unknown.

HOW TO GET THERE:
Several airlines fly twice weekly services to Tonga's capital of Nuku Alofa. From here you can catch in a short flight to Neiafu – in the northern Tongan island group of Vava'u. In Neiafu you can book a yacht or motorboat to take you out to Nua Papu island, where Mariner's Cave is located.