It's enough to stun the kids into uncharacteristic silence.
Then a small voice breathes, "Woah!"
Nate, 7, clutches my hand as we tread carefully down, down, down into Waitomo Cave. He's goggle-eyed at the softly lit yellow-white limestone ceiling, stretching into a gigantic arc, and the ground falling away beneath the steps.
The enormous scale of this ancient cavern makes us feel like hobbits venturing furtively below ground.
Massive tooth-like stalactites drip water and minerals to create stalagmites below. Our guide points out formations that resemble a family complete with Rocky the dog, a parachuting kiwi and Bob Marley's dreadlocks before wryly quipping that she's obviously spent too long underground.
She's captured the imagination of the kids, though, who see a rock elephant and a cocker spaniel. They're delighted to learn the nifty mnemonic: "stalactites hang tight to the ceiling and stalagmites might reach the roof one day".
Some are close to joining to form a column. It's an exciting prospect as these rocks take 100 years to grow just one cubic centimetre.
We're silently fizzing with excitement as we shuffle further underground to the boats. Once we are seated, our guide insists on absolute quiet.
He shines his torch at one visitor and deadpans, "If I fall in, you're in charge," before pushing off from the jetty into the inky black of the glow-worm grotto. We're enveloped in darkness and it's a moment before I remember to look up.
We're beneath a magical galaxy of white-blue lights - each bright pinprick surrounded by a soft haze. The cool, dark open space gives the surreal sense of gliding on a magic carpet beneath a velvety night sky studded with stars.
Nate hunches close to me, worried he'll get sticky strands in his hair, but soon relaxes when he realises they're up out of reach.
Our guide doesn't give commentary, he simply pulls us along the cave using ropes. All we hear is the soft slap of water and it's as if time has paused.
The spell is broken when we see daylight and emerge blinking into the lush green forest.
A short drive up the road is Ruakuri Reserve with its spectacular bush walk featuring bridges and limestone cliffs. Waitomo Stream meanders lazily beside us before thundering over - and th own into the darkness to a lookout.
The stream is rushing below and there. Among the stalactites above is one lone glow-worm. The kids can't wait to see more of them in Ruakuri Cave.
The longest underground tour in Waitomo, Ruakuri has a jaw-dropping dramatic entrance, a massive 15m-high spiral ramp.
This impressive engineering feat makes it wheelchair accessible and gives the feeling we're on a journey to the centre of the Earth.
It's a two-hour tour past diverse limestone formations helpfully named for what they resemble: "curtain" looks like swathes of cloth, "bacon" has streaks of colour.
Each section has its own lights so we are treated to a big "reveal" after our group arrives in darkness. Hundreds of glow-worms are above and the water below is sparkling.
"The name glow-worm in Maori translates to reflection on the water," our guide, Meike, tells us.
"They're actually more like maggots than worms," she laughs: "But that name would be hard work for the marketing department."
• The writer and family travelled courtesy of Discover Waitomo.
Need to know:
•There's no supermarket in Waitomo, so take supplies from home, stop in Otorohanga on the way or enjoy the local eateries.
Modern and airy with a brilliant view of rolling hills from the deck. Crispy buttermilk chicken and mini lamb burger got the thumbs-up from a great kids' menu, the adults were happy after rump steak and lamb shoulder rack, and a beer from King Country Brewing Company.
•Long Black Cafe:
At the home base of the Black Water Rafting Company, with people in wetsuits strolling through. We love that kids' icecream cones are just $1.60.
•The General Store:
Dating back to 1910, this is the original one-stop-shop but is now more of a cafe. We loved the eggs benedict with potato cakes, spinach and salmon.
This pub burned down three years ago but in the new bar we loved the steak and stout pie.
•Waitomo Caves cafe and restaurant:
Despite being regularly slammed with large tour groups, the staff are efficient and calm. The arched canopy of sail cloths is spectacular. The glow-worm snack box is good value.