One of the downsides of taking a grandchild caving is that they can slip through gaps that larger, older, creakier bodies find a bit ... tricky.
My 8-year-old granddaughter Kate found the walk up Rangitoto's volcanic cone to the lava caves a bit taxing. "I've got smaller legs than you, granddad."
But once we reached the caves themselves she was in her element. "I've got more to fit through than you, Kate."
At the first rather small cave I gave up when I saw the narrow cleft she had vanished through and walked round to meet her at the other end.
The biggest of the lava tubes, which must be close to 100m long, was rather more my size, and thanks to a skylight in the middle you can get through without a torch ... which is just as well because Kate had grabbed mine and zoomed confidently ahead.
It was almost comfortable enough to live in, and what looked to be a sleeping bag, with a smell of urine nearby, suggested someone was doing just that.
You definitely did need a torch in the third cave we went through, which had no skylights, a narrow entrance and exit, and involved lots of scrabbling through narrow gaps in the pitch black.
Once again I was left behind in the dark, this time to be rescued by a delightfully noisy Italian family, who had lots of lights and were having a great time in this underground fun park. "Is this where the molten lava came through?" asked the mother of the family. "It must be," replied a daughter.
"Ah," said mother, "then I keep wondering what happens if the volcano erupts again while we are down here."
"Well," I said, always happy to ease the gloom with a little sunshine, "volcanoes seem to occur in Auckland every few hundred years, and it's about 600 years since this one went off, so we're about due another eruption."
"Perhaps we should hurry then," the mother said cheerfully. "Shine the lights over here so we can see where we are going."
With the aid of the many lights which suddenly shone in our direction we were able to get out of the cave before the next surge of magma from the middle of the earth.
These are not the most amazing caves I've ever been in - there are no stalactites, glow worms or bats - but they are good fun and they are all part of what makes a visit to Auckland's iconic island an interesting experience.
When I was a youngster our family spent a lot of time on Rangitoto, because my Uncle Colin owned one of the island's baches.
As well as the volcanic cone itself, with its awe-inspiring crater and those lava tunnels, there were countless little bays where you could easily imagine pirates landing; a myriad little caves, just the place for hiding treasure, in one of which we found the skeleton of a wallaby; the scary remains of the camp used in the 1930s by the convicts (ex-pirates?) who had the hellish task of carving a network of tracks and roads out of the rock; the mysterious kidney fern glade; Wreck Bay where a dozen ships were abandoned (pirates again); and the most fascinating flotsam and jetsam I've ever seen (once we found a naval officer's hat).
It must be 20 years since I was last there, but my visit confirmed that it's still a terrific place to explore, though some things have changed over the years.
One obvious change is the absence of most of the baches, demolished by official decree 20 or 30 years ago, and the demise of store and tearooms run by the legendary Reg Noble (we called him "Big Chief Noble" because my Dad told us he was the hereditary chief of the island) and Aunty Vi Leech.
And instead of Reg's ancient bus, transport round the rocky roads is provided by Fullers' tractor-powered train, the Volcanic Explorer, which offers a tour of the main highlights complete with an excellent commentary.
But the biggest change of all is a result of the Department of Conservation's successful campaign to eradicate possums and wallabies.
These Australian migrants were once very much a part of island life. When baches were occupied, wallabies used to gather round the door in the early evening hoping to be fed. And one of our family legends involves the time my parents went to check on their little darlings and found a possum asleep at the foot of my bed right alongside my little brother's pudgy head.
Unfortunately any romance provided by the presence of these furry creatures was heavily outweighed by the damage they did - it's estimated that there were 20,000 possums on Rangitoto each consuming 6kg of vegetation a night - and now they have gone, the plant life is thriving.
The climb to the crater at the top, once up rough rock tracks through a bare moonscape, is now along a boardwalk through a flourishing forest, which even has a few tuis and fantails.
The trees at the top are getting so high they have to be trimmed to stop them blocking out the magnificent views of Auckland City and the Hauraki Gulf from the summit. And the crater, which in my youth was so bare you could easily imagine it had erupted only a few months before, is now a green shrub-lined bowl. In fact, it's now disappointingly benign. I had to agree with 4-year-old grandson Geoffrey, who had been eagerly anticipating a sight of the crater which once spewed forth red-hot destruction, but looked down into the tranquil verdant basin and said contemptuously, "That's not a 'cano."
Fullers' Volcanic Explorer package, including the boat trip and tour, is $50 for adults and $26 for children. The boat trip on its own costs $20 and $11 respectively. See www.fullers.co.nz or ring 0800 FULLERS for bookings.
It also worth looking at www.rangitoto.org, the website for the Rangitoto Island Historic Conservation Trust, made up of the old bach families.