Underneath our biggest city lies an extensive network of lava caves.
These can be found in the most unusual suburban places – backyards, under manholes and beneath school grounds – but due to increased urban development, they're under threat.
Speleologist Peter Crossley has spent five decades documenting over 250 of these underground spaces.
He tells Nine to Noon's Kathryn Ryan he's either catalogued, found or been into every cave in metropolitan Auckland.
Crossley says his love of caving may date back to his English childhood in the 1940s.
"My mum had a homemade cake shop. When the air raid sirens went, she'd put me into the cold ovens for safety."
Cavers aren't fearless in enclosed places, but fear is part of the excitement, he says.
Unlike claustrophobics – "an unreasoned fear" – most cavers have experienced the reality of a jam.
"They know what it's like if you get stuck … Yes, I have been stuck. I think all cavers have been stuck, but very few permanently.
Cavers need to be "cold fish", Crossley says.
"You've got to have a reptilian mind and be able to clamp down on all emotions in a cave situation where you're a little bit stuck and you can't go straight up to get to the air."
A sense of adventure is also required.
"You go in there and it's that dark passage leading off and you don't know what's at the far end of it. For a caver it's almost impossible to stop. You just see that darkness, or whatever it is, and keep on going."
Crossley doesn't advise non-cavers to go looking for caves, especially on Rangitoto Island, where the vegetation that has sprung up since the removal of wallabies and possums makes them much harder to find than in the past.
"Once you're off the tracks it's really quite dangerous, and there are holes and chasms which you could fall into."
So how were Auckland's lava caves created?
"Imagine a volcano, and it's erupting and there's lava spewing up and running down the hillsides - Rangitoto, the Aucklanders can think of that - as the lava flows down the hillside the top crusts over, and the lava continues on down inside it, underneath that crust. Eventually, the lava stops flowing, the middle drains out and you're left with a tube."
The lava caves in Auckland are up to 1 - 2.5 metres around, Crossley says. This is modest compared to Hawaii, where lava flows for months or even years create caves of up to 10 metres.
And while Auckland's caves can be as long as 200 or 300 metres, the longest in the world (on Hawaii's Big Island) is about 50 kilometres long and just over 1,000 metres high.
The walls of the caves can be very colourful, he says – "Imagine a room with rimu panelling around the sides with black and white streaks." and in leafy areas contain tree roots.
Many of the caves predate the arrival of Europeans, and some feature in Māori legend, including one in what is now Mount Albert.
When a tribe in the area where besieged, they ran all into a cave overnight. All but the chief burrowed through a hole in the back of it and escaped to Meola Reef.
In the early 1900s, a house was built over the entrance to the cave, Crossley says, and in World War II this was used as an air raid shelter.
Almost all accessible caves were used as burial sites by the early Māori, says Crossley, but there is little evidence of this now. Many of the remains were looted in the 19th century – some even removed and ground for fertiliser.
Auckland's lava caves have historic value for both Māori and pakeha, he says.
"I hope that, with others, I have got the council recognising that they're part of Auckland's heritage, because they are … I hope that I have managed to at least record them so no more will be destroyed."
A short documentary about Peter's life's work called Meet Peter will screen in Wellington and Auckland next month as part of the Doc Edge International Film Festival.