It turns out magma lurking beneath the Bay of Plenty is to blame for thousands of small earthquakes which rocked the region between 2004 and 2011.
Initially scientists believed the shifts in the tectonic plates were to blame for the tremors but the blame belongs to something a bit hotter.
Nine kilometres below the coastal town of Matata lies a previously-unrecognised magma chamber, scientists say.
Molten rock was being shifted and pushed up, deforming and cracking surrounding rocks and raising the land around 40cm since 1950.
The movement caused several thousand small earthquakes in the region. Most of them were between 2km and 8km deep, and between magnitude 2.0 and 4.0.
Published in the Science Advances journal, the modelling suggests the magma is slowly growing -- by about the size of 80,000 Olympic swimming pools since 1950 (200 million cubic metres).
This discovery doesn't change the risk of volcanoes in the Bay of Plenty region, the researchers say, and doesn't suggest an imminent eruption.
"While there is absolutely no evidence pointing to volcanic unrest in coastal Bay of Plenty, this finding underlines the fact that we live in a geologically active country where it pays to be prepared," says lead author Dr Ian Hamling from GNS Science.