Caves created by rivers of lava underlie New Zealand's biggest city. A new research project is documenting Auckland's lava caves, hoping to protect this hidden geological heritage and understand what future eruptions might have in store.

Known for its iconic maunga like Rangitoto and Maungawhau Mt Eden, Tāmaki Makaurau Auckland is a city built on an active volcanic field that has erupted at least 53 times.

But beneath streets, houses and parks, there are other - hidden - remnants of the city's fiery past: hundreds of lava caves.

The backyard cave

Lava caves form when hot flowing lava meets air and crusts over, creating a tunnel. Eventually the lava drains away, leaving behind a cavity.

Formed as far back as 200,000 years ago and as recently as 550 years ago (when Rangitoto erupted), lava caves in Auckland range from small cracks to lengthy tunnels. The longest, located in Wiri, stretches to 290 metres.

The lava cave in Sean Jacob's Mt Eden backyard is about 100 metres long. "For something that's so quiet and so peaceful when you're down here, it was sort of created by so much violence," he says.

The Jacob family bought the property in 2008 - in part so the cave would be protected, unlike many others across the city which have been destroyed or infilled with concrete in years gone by.

The speleologist

Peter Crossley is perhaps the only person who went inside some of those caves that no longer exist. A speleologist, Peter has spent 50 years documenting Auckland's lava caves.

"Some people would say that they're muddy, grotty, dark, infested with rats and all the rest of it. But when you look at it, you realise: it's a tunnel which has been filled with lava, almost yellow in heat, that could frazzle you in a microsecond," he says.

Over the decades, Peter has seen surveying methods advance from compasses to state-of-the-art 3D scanning, giving scientists unprecedented detail and valuable insights into past eruptions.

Now he's passed on his knowledge of 180 lava caves to a new research effort.

A new lava cave every month

Jaxon Ingold, a master's student at the University of Auckland, is collating everything we know about Auckland's lava caves - drawing on Peter's records, historical sources, and mātauranga Māori - so this geological heritage can be better protected and respected.

"What I'm currently working on is: is it possible to predict where as-yet undiscovered lava caves may be located? So that we can be more careful in those areas," says Jaxon. …

Go to this episode on rnz.co.nz for more details