Karl's voice floats upriver through the echoey dark: "Basically, what we're looking at is the snot and poo from a bunch of cannibalistic maggots."
A chorus of "Ewww!" bounces off the rock walls either side of me; Mel, Nicole, Sam and I suddenly disenchanted by the mass of twinkling glow worms on the cave ceiling above.
Karl's explanation may not be the most scientific (or the most charming), but it certainly is accurate. "Glow worms aren't actually worms at all - they're larvae," he explains. "The light comes from their waste, and they use it to attract insects. Glow worms begin eating as soon as they're born. If there's nothing around to eat, the first one to hatch will start eating its brothers and sisters!"
Keen to escape the suddenly grotesque glow worms we switch on our headlamps, slide our bums off the rocks back into our rubber tubes and continue to float downstream through the Ruakuri Cave.
We're at the beginning of the Black Labyrinth - one of the many cave adventures offered by The Legendary Black Water Rafting Co. in Waitomo.
At three hours in length, the Black Labyrinth allows for the long, hilarious process of wet-suiting up followed by a short, pleasant bush walk and just over an hour's cave time - jumping backwards off waterfalls, gaping at glow worms and posing for photos. At the end, a hot shower, soup and toasted bagel await.
Just 15 minutes in to the cave that hot shower seems a long, long way off.
"Ready Amelia?" says Karl as, standing, I edge backwards with the rushing river towards the top of the waterfall. "Ummm... sure," I stammer. Holding the rubber tube firmly on my backside I take a deep breath and - ignoring every screaming particle of common sense that I possess - jump backwards, down the waterfall into the darkness.
I land with a scream, a "splosh" and some cheers from my fellow Labryinthers. Looking back, the waterfall is pitifully short. However, abuzz with our accomplishments we form a long snake, hooking feet under armpits, and float determinedly onwards.
Thankfully, it's not until later that day that I find out about the eels that reside in Ruakuri Cave. "Ten feet long," I overhear one guide saying. "Wrapped itself around the guide's leg," says another. Had I known this beforehand I wouldn't be quite so relaxed as I bob downriver in the darkness, blindly grasping what I assume is slimy, jutting limestone and dangling my feet in the cool, rapid water. I probably wouldn't have dared stop my tube either.
"Jump off here!" calls Karl. Four headlamps create a criss-crossing laser show as we all try and master the simultaneous art of slipping off our tube, catching it before it floats away, getting our footing on the slippery floor and bracing ourselves against the flow of the river.
Nestled against the rock wall once again we look up. Far, far above, through the dripping water that sploshes on to my forehead, I see a light. "That is a tomo," explains Karl. "'Tomo' is Maori for 'hole'."
I knew I was underground. For starters we had to enter the cave through a near-invisible gap in the rock that I would never have noticed had Karl not pointed it out. Once inside the air became thicker and in the cloying darkness I could see the steam puffing from my mouth at every exhalation. The sound of trickling then rushing water filled my ears as I ventured across slippery rocks, further below ground, walking like an old man to avoid the stalactites aiming towards me like upside down ant hills. All of this told me I was underground, yet when I look up at the shining tomo 65 metres above I'm shocked at how far down we are.
My panic is fleeting, warded off by Karl's call to remount our tubes and float on.
With our lights off again and the river slowing to a gentle meander the cave is filled with a dense silence, punctuated only be the dripping of water into water. We don't speak. We don't touch. For all I know the others have disappeared - escaped into a secret side cave or paddled off without me.
Strangely, I'm unworried by this scenario - content with the tranquil darkness and smooth river. I recline in my floating rubber tube and tip my head back, eyes to the ceiling. A Milky Way of glow worms stretches out above me. Their glittering beauty is accentuated by the black silence and the surreal, serene feeling of drifting through a cave. Paddling with my hands I spin my tube around, looking back the way we've come. Thousands of tiny lights twinkle back at me. I'm floating through outer space.
All too soon a soft natural light begins to ripple across the river. It gets gradually brighter, interspersed with the long shadows of stalactites hanging from the cave's exit. I don't want the daylight. I want to continue floating gently through the quiet trickling cave with only clusters of cannibalistic maggots to show me the way.
Amelia is Content Editor for the New Zealand travel and tourism website