A Ngaruawahia resident has captured remarkable footage of an unexplained water eruption at the Waikato River.
Martin Smith posted the video to Facebook yesterday afternoon, asking followers to help distinguish the mystery behind the bubbles.
"WTF is this?" he wrote in his caption.
"I have been living in Ngaruawahia for over 8 years and spent a lot of time at the point bow fishing but have never seen anything like this before."
Mr Smith can be heard in the video saying the river had been bubbling since yesterday morning.
Facebook users replied with a variety of possible reasons, ranging from "methane bubbles" to the elusive "taniwha".
The video has been shared more than 2,000 times since it was uploaded and viewed almost 100,000 times.