Maya Wei-Haas

Reporter

In a suburb of Auckland, New Zealand, a lush, almost tropical oasis hides behind the home of Sean and Annie Jacob. Palm trees tower over the verdant lands. Cheery marigolds blossom in raised garden beds. And water invitingly ripples in an elevated pool. But the reason for my visit to the Jacobs’ home lies beyond these botanical riches, down a set of earthen steps that lead to a gaping entrance into Auckland’s volcanic underworld.

The sprawling cavern, formed by flowing lava some 28,000 years ago, is just one of more than 250 caves documented beneath the bustling metropolis. They were left by eruptions of 53 volcanoes that laid Auckland’s foundations. Most of the caves formed from the flow of ancient lava. The edges of the molten stream cooled first, creating the cave’s rigid walls and ceiling while insulating the rush of lava within. When the eruptions ended, the tubes drained, leaving behind snaking volcanic caverns.

Some of the caves are just small crawl spaces, allowing only a few steps in any direction, while others—like the nearly 350-foot-long tube that opens into the Jacobs’ backyard—may be arms of once-larger networks.

Auckland’s lava tubes hold great spiritual significance to the indigenous Māori people, who see their origins in the volcanic landscapes. “We are the descendants of these mountains and volcanoes,” says Kelvin Tapuke, a senior research fellow at Massey University, who belongs to the Ngai Tai ki Tāmaki Makaurau tribe in Auckland. Lava flowing from the peaks—and the caves that later form within—carries with it a lifeforce, known as mauri, “your essence of who you are,” Tapuke says.

But these relics are now at risk as the city continues to grow. In 2023 alone, the net number of people migrating into New Zealand was 127,400, with many choosing Auckland as their new home. Numerous caves have already been lost. Construction crews filled many of them with cement while erecting new buildings and laying new roads, while miners destroyed others through quarrying. In a bid to help protect these priceless pieces of cultural and natural heritage, a new effort, supported by the multi-agency project Determining Volcanic Risk in Auckland (DEVORA), aims to compile the most complete database yet of the city’s volcanic tunnels.

Inside an Auckland Cave
Sean and Annie Jacob have worked to restore the cave to its natural state, removing piles of trash such as bricks and broken glass. Maya Wei-Haas

Studying Auckland’s volcanic caves could help give clues to the region’s volcanic future, explains speleologist Peter Crossley, who has led many efforts to map and protect the region’s caves since the 1970s. While the city’s 53 volcanoes are dormant—only one ever erupted more than once—molten rock is still brewing underground. “We’ve got a damn near 100 percent chance that there will be another volcan[ic eruption] in Auckland,” Crossley says. “But we don’t know where or when.” Studying the caves could help geologists better understand past flow of lava and thus where the molten rock may travel in the future, he explains.

While support for protecting the caves has grown in recent years, it faces a huge challenge: No method can reliably map Auckland’s subterranean passages from the surface, apart from boots-on-the-ground exploration. But Auckland University master’s student Jaxon Ingold, who is leading the effort to create the new cave database, hopes to turn a high-tech eye to the challenge by using machine learning to predict cave locations. The models use cave data collected from decades of exploration by Crossley and others to connect landscape features like ground slope to past cave formation, and thus pin down spots where undiscovered hollows may hide.

By the light of his cellphone, Sean Jacob carefully guides me through the cave’s silent darkness, pausing to remark that we just passed under the road where I parked my rental car. We’re now standing under the house across the street. In 2008, he and his wife purchased their neighbor’s property, cave entrance and all, to protect the geologic cathedral from entombment under new construction. They’ve since worked to restore the hollow to its natural state. Surrounding us are clues to the cave’s molten past, including small rocky droplets on the ceiling that look like they’re just about to fall—drips of lava frozen in the act.

Auckland Cave Ceiling
Swaths of the cave ceiling are covered in tiny droplets of stone that formed when the system was piping hot. Maya Wei-Haas

Many of Auckland’s lava tubes have been known for as long as people have lived in the region. Some iwi (a Māori tribe or nation) historically interred the bones of their ancestors in the caves along with precious taonga (property or treasure) like necklace pendants and earrings. Every year their descendants returned to rub oil or fragrance on the bones and talk with their ancestors, Tapuke says. In the 1800s, Europeans, Australians, Americans and others plundered the caves’ riches, selling them to collectors and museums—where many still remain. Repatriation “is still an ongoing thing,” says Tapuke, who also works with DEVORA to develop culturally responsible practices.

For many years the caves had little protection, and few settlers knew of their full extent. When Crossley first started exploring Auckland’s caves in the 1970’s, only a couple dozen had been formally documented. An undergraduate at the time, Crossley joined a small group of avid cavers who would venture to different neighborhoods and knock on doors to politely ask residents if they had a cave in their backyard. Often the answer was yes: “We were finding new caves almost every weekend,” he says.

Over the years the caves served many purposes, including as a hideout for an underground communist printing press during World War II and a mushroom farm. As the city has grown, so too has the number of clashes with the caves. Construction crews frequently break into concealed volcanic pockets while laying new utilities and roads or raising new buildings. Once an excavator even crashed into a lava tube. The caves can also be unintentional conduits, says Kate Lewis, a geoheritage specialist on Auckland Council. Heavy rains and floods hit Auckland last year, filling the caves with water that burbled up into the backyards of unsuspecting residents.

In many of these cases, the caves have emerged the worse for wear. People used some caves as personal dumps, choking the subterranean passes with refuse. One such pit lies in the middle of what is now a bull paddock in south Auckland, where I meet Ingold. We delicately pick a path through the uneven terrain as a bull menacingly stares at us from across the grassy expanse. “Some of these rocks aren’t rocks,” Ingold cautions as I hop over a particularly mushy cow pat.

Auckland Cave Exit
Some of the caves are just small pits, like this one Kate Lewis exits in south Auckland. Maya Wei-Haas

The cave entrance is just large enough for a person to slip through, covered by a wire fence laid flat on the ground to prevent unwanted visitors, both human and bovine. A few steps down a ladder and we’re standing in a small subterranean pocket whose size would make a decent walk-in closet but feels like a cupboard compared to the yawning chasm in the Jacobs’ backyard. While small, the cavern serves as an important educational tool for students and other sanctioned visitors to learn about the caves. Ingold points to a pile of trash at the ladder’s base. “People were just chucking stuff in there to get rid of it,” he says. While most of the trash has been removed, a few artifacts—a glass bottle, a rusty chain, a few random chunks of metal and a pair of dilapidated boots—remain to teach how not to treat the caves.

Creating a cave database has risks, Ingold acknowledges. Exposing the location of all known caves could increase vandalism and visitors with no respect for the significance of these features. He’s currently working to develop systems to limit who can see sensitive info, like cave locations. Instead, he hopes the database is a resource for Lewis and her colleagues as they work to reverse historical trends of disregard for the caves.

As part of her position with Auckland Council, Lewis works with developers, builders and landowners to foster awareness about the significance of the caves for local iwi. Crews are now required to report any lava tubes they tap while building. Then Lewis and her colleagues work to develop solutions to preserve the caves without hindering construction, such as rerouting utilities around subterranean cavities or installing manholes in overlying sidewalks to maintain access. “It’s an interesting balance, because we are really trying to protect the caves,” she says, “and they are really interested in finishing their project.”

Lewis and her team try to be quick to respond, developing creative solutions with the hope that builders continue to report new caves rather than filling them with concrete, as was common in the past. Ingold’s database could facilitate these efforts or help guide future construction by clueing developers in to potentially cave-rich regions.

“There’s a need for mindfulness,” Tapuke says, stressing the Māori’s deep connection to whenua, a term for land. Yet the word’s full meaning is much more expansive: “Whenua is not only the land, but it’s also the umbilical cord of every person,” he says. “It really is the food source that gives life.”

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Maya Wei-Haas

Maya Wei-Haas is a freelance science writer who specializes in geology of Earth and beyond. Her work has been featured in National Geographic, News from Science, and AGU’s EOS.