The climax of our leviathan trip was one of the most dangerous and dramatic expeditions I've been lucky enough to partake in.

Half of the team had headed to Indonesian Borneo and were following my course up the Segah River, searching for the fabled black orangutans and exciting invertebrates. Hopefully, they would not suffer as I did with the boat disaster I described at the start of this blog.

My half of the team was heading to the extraordinary Mulu Mountains in Sarawak in search of caves and cliffs, of which we found plenty.

The only real way to get around the Mulu National Park is by helicopter. The pointy mountains are constructed of rocks so sharp you could shave an orangutan with them, and anything less than vertical is encased in spiny, spiky, just plain awkward jungle.

Up in the whirlybird, though, you can get a big picture of the place and it'll take your breath away. About an hour into our second flight we hovered over one of the park's larger peaks, and all of a sudden, became aware of a dark hole in the forest summit. It was as if someone had tossed a black M&M down onto a green carpet.

As we got closer it became evident that the hole was probably wider than a football field is long and dropped many hundreds of feet into the bowels of the mountain. Even better, a vast white stalactite, possibly six stories high, hung from its lip, pointing down into a cauldron that was green with vegetation.

A miniature ecosystem shut off from the outside world for a million years — we knew instantly that to explore its depths would be the finale we were searching for.

The team hacked a path up a vertical mountainside to the lip of the sinkhole — what's known as a doline — where a humongous underground cavern's roof had collapsed, opening the hole to the world above.

This area of startling limestone mountain systems in eastern Sarawak is peppered with unexplored cave systems due to its karst construction and contains the largest caves in the world. One of the caves is so big you could fit five St. Paul's Cathedrals inside or 40 jumbo jets wingtip to wingtip.

Our own sinkhole was more modest at approximately 1,000 feet deep from lip to bottom and spacious enough for only one decent football stadium. That said, it was one of the most impressive places any of us had ever been and we were the first to ever venture into its deepest caverns.

I spend a good deal of my life hanging off ropes and no one would ever accuse me of being nervous around heights, but when I stepped off into space with that unimaginable void beneath my feet, the world started to spin and the nauseous whirl of vertigo leapt right into the roof of my mouth. The camera clearly records me stating my reaction for posterity — "I want my mum."

Beside me, the vast white stalactite hung like the bony finger of a giant Grim Reaper, bright yellow orchids glimmering at its base. I was just trying to do anything other than look down. At one point, my rope slipped slightly from the tree it was fixed to above me and I emitted an involuntary squeal, which warbled around the cavern like Mariah Carey in a blender.

There was a genuine tremor in my chest as my boots touched ground deep inside that majestic cavern, stepping into a startling lost world that had been sealed off from the outside jungles since the roof above collapsed a million years ago.

And WHAT an ecosystem it was, vastly different from the forest above with prehistoric ferns and snake-like, spindly trees sprouting from lethal razor spears on the limestone floor. Single-leafed palms followed the sun's brief progress around the floor like radar dishes and vines hung like dreadlocks from the rock walls.

We made our base in a sandy-floored cave protected from the endless rain by a stalactite-decorated ceiling, which seemed to be aiming those giant rock harpoons down at our heads.

In the dark cave systems beyond I caught an albino gecko, which sat on my hand licking grit from its eyeballs as I watched blood pulse from its heart through its translucent skin. A flighty stick insect spread its colorful wings in a threat display, then dropped its head like a pawing bull only to reveal a luminous yellow dog collar around its neck.

We also saw creatures from any bug hater's nightmare: venomous centipedes with immense, long spindly legs (Scutigera) scuttled around the walls at a horrifying pace, and grotesque cave crickets with antenna as long as my arm and wicked sickle-shaped ovipositors protruding like scimitars from their rear ends invaded our camp.

We spent three nights on a sandy-floored bed at the base of the sinkhole — our next objective was to find a way out. In one corner, the hole funneled downward into a jumble of vast boulders, and unbelievably, the whole gigantic feature was exited in the bottom by a tiny hole barely bigger than my shoulders, which led us down ropes into a succession of tight squeezes into the darkness.

I am not a big fan of caving. Blue skies and big views are my potion, not squeezes through tunnels you may never be able to exit, heading deeper and deeper into the bowels of a mountain. One particular squeeze was so tight it could only be achieved by Yogic contortion, whilst breathing out —  it was terrifying. That night I woke in a cold sweat, dreaming of being stuck fast in a pipe-like hole while cave crickets and Scutigera chewed out my eyeballs.

Every new tunnel was illuminated for the first time EVER by our torches (flashlights) — we had to move painstakingly slowly to avoid snapping brittle and perfect stalactites. There would really have been no hope and no motivation to push on, except for one thing — wind. A dry, warm breeze howled up from the giant caverns below, leading us irresistibly on like Frodo into the depths of the dwarfish mines.

Oddly enough, Tim Fogg (my caving partner and not a bad person to have along as he was pretty much solely responsible for charting and exploring the caves of the region) exacerbated the Tolkienesque illusion as he bounded along on all fours, his big, icy eyes cursing the dreaded sunlight we left behind and drawing us on into the darkness, "Come, hobbitses, come, we shows you the way, we does."

On the second day of exploration in the deep, Johnny the cameraman took a slip in the cave, plummeting forward and twisting his ankle. My (ridiculous) reaction was to run quickly to his aid, slipping even worse and disappearing straight over the edge of a 12-foot drop, bouncing flat on my back halfway down and landing in a pile of sand.

Anywhere else in this brutal scenery of rock harpoons and razor edges and the result would have been ...well, let's not even go there. Johnny was evacuated — hoisted out of the hole on pulley systems. Tim decided we would give the cave one more day to pop through then turn back.

At about 4 in the afternoon, a narrow tube with air roaring through it like a wind tunnel opened out into a series of breathtaking underground cathedrals. The first was separated in the center by two vast towers — each cavern could have contained a passenger ferry. Ours were the first eyes ever to pierce the darkness.

We wandered awestruck for an hour or so, but the wind had vanished. It would have taken months to explore every side tunnel and hole. We turned back, hearts heavy, yet at the same time profoundly moved by what we'd seen.

A week later, sitting in a local hut (knocking back rice wine and doing our best to emulate the vibrant native dances to please our hosts), shimmying up our ropes was a forgotten irrelevance. All we could see in our memories was one of the most beautiful places in the world — and we were the first to explore it! It was a privilege not many experience in this shrinking modern world, and something we will all treasure for the rest of our days.