The New Zealand Defence Force's role in guarding against threats has taken it to the bottom of Lake Rotorua.
The Navy's Littoral Warfare Unit is surveying the lake floor to identify volcanic and geothermal features that could be a risk to lake users.
Lieutenant Commander Tim Garvan says the results of the bathymetric and magnetic survey will provide GNS Science with a high-resolution map.
"The map will enable scientists to identify areas of past hydrothermal activity at the bottom of the lake and locate active thermal vents," he said.
"The survey will also show areas of shoaling which are potentially hazardous to vessels and sailors."
GNS Science marine geologist Cornel de Ronde said the lake sat within the Rotorua Caldera, which was in effect a large volcano.
He said the five-and-a-half-week survey, expected to be completed in late November, was the first in a series aimed at determining how much heat was being discharged from the volcano.
"Ultimately that feeds into our existing hazards assessment of the area."
Dr de Ronde said the map would be a virtual three-dimensional image of the lake floor.
Among the features it would show would be the location of a significant hydrothermal eruption crater that appeared to be active.