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.