There's no mystery behind a light plane seen circling the skies over Rotorua on Thursday night. It was part of a scientific survey studying Rotorua's geothermal activity.
Taupo-based GNS Science conducted an aerial thermal infrared survey which involved measuring ground temperatures with a thermal infrared camera.
They had to wait for a night with fine, clear skies to undertake the survey.
Some Rotorua residents said they were disturbed by the noise of the light plane flying about 1200m above the city. More than 150 comments were made on the Rotorua Daily Post Facebook page about the flight, some of them complaining about the noise.
Many had theories for why the plane would be flying over the city so late at night.
These included that it was looking for dope, it was lost, a pilot training exercise, spraying chemtrails, and that it was a UFO.
The truth? A public notice in Tuesday's Rotorua Daily Post stated:
"Aerial Thermal Infrared Survey - GNS Science will be conducting an aerial thermal infrared survey in the Rotorua area one evening within the next four weeks between March 4 and 31. The survey involves measuring ground temperatures using an thermal infrared camera. Results from the survey will be used for scientific purposes. The survey involves using a light aircraft flying at an altitude of about 1200m in the vicinity of Rotorua township.
"The survey is weather dependent and is scheduled to occur between the hours of 9pm to 3am. Noise from the light aircraft is not expected to affect residents in the area."
GNS Science geophysicist Rob Reeves said he apologised for any disturbance caused by the flight.
"We do hope to do it again in the future, but not in the near future. It's not something you can do every year, but ideally we want to do it again in five to 10 years.
"It certainly was not our intention to keep people up ... what we were trying to do was remove any solar effects to measure heat coming naturally out of the ground," he said.
Mr Reeve said in future they would look at using a quieter aircraft.