Researchers and Auckland Council are collaborating to map a field of hundreds of lava caves under the City of Sails in Auckland. The project aims to create a comprehensive database on the caves, as homeowners and developers are discovering approximately one new lava cave every month. The caves are formed when the outside layer of lava cools down fast enough to harden, creating a tunnel for the lava to flow through. Many caves are considered important to iwi heritage, and any cave over one meter wide will be investigated and potentially protected. The last caves were formed 600-700 years ago during an eruption on Rangitoto Island.