Stewart's Cave in Mt Roskill is one of the many lava caves hidden beneath Auckland's suburbs. Photo / Paul Rowe
Stewart's Cave in Mt Roskill is one of the many lava caves hidden beneath Auckland's suburbs. Photo / Paul Rowe
They’re among Auckland’s most unusual and intriguing features – and are still being discovered beneath the city at a rate of one a month.
Now, researchers are setting out to map hundreds of underground lava caves, in a major new project to help inform future development in the city.
It’s well-known Auckland is built upon an ancient, sprawling and potentially active volcanic field spanning more than 50 centres – but fewer city residents might be aware of the extent of lava caves lying not far beneath their feet.
They’re found in few other places in New Zealand and are considered relatively rare even by world standards.
Among the largest is a massive system running 250m beneath Kitenui Avenue and surrounding streets in Mt Albert, and accidentally discovered in 2006 by workers repairing gas pipes.
The caves were formed when a specific type of erupted lava flowed at a speed that allowed the outside layer to cool down fast enough to harden.
Auckland's underground lava caves - like this one which featured in a 2017 exhibition - are considered globally rare. Photo / Chirag Jindal
“This creates a tunnel for the lava to flow through, and when the lava drains away a cave is left behind,” University of Auckland researcher Jaxon Ingold explained.
With Auckland Council geoheritage experts Kate Lewis and Christina Bloom, and veteran speleologist Peter Crossley, Ingold now aims to create a complete database of the city’s known caves.
“Many people know there are lava caves underlying the volcanic parts of Auckland, but the exact information is spread around lots of different groups, like local iwi, the council and experts who have been investigating and mapping some of the network,” he said.
When discovered, new lava caves were reported to council, which helped find solutions for completing planned works with minimal impact on them.
“Our team has become extremely responsive to the needs of anyone reporting a new cave during works in order to find options to continue the project, however, some modifications to the works methodology and project are typically required,” Lewis said.
“We help them develop construction methodologies to reduce vibration and work carefully around the cave, as well as working with geotechnical engineers, where required, to re-design structures over and around caves safely.”
Speleologist and project member Peter Crossley, pictured in a lava cave beneath the streets of Mt Albert. Photo / Sylvie Whinray.
Under the Auckland Unitary Plan, freshly found caves measuring more than a metre wide needed to be investigated and potentially protected – and many were scheduled as Outstanding Natural Features due to their cultural, historical and geological significance.
“We want to encourage Aucklanders to report new caves and share any information we may not have,” Lewis said.
“There are several reasons why this information could be sensitive, so we want to reassure people we will treat that information appropriately.”
For mana whenua, even caves that hadn’t been exposed to the surface were culturally important.
“Because these sites are incredibly significant for local Māori, we recognise the importance of working closely with local iwi to get a better picture of these sites,” Ingold said.
The effort was being led by Determining Volcanic Risk in Auckland (Devora), a long-running research programme funded by Auckland Council and Toka Tū Ake EQC.
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
