The mystery behind a bubbling phenomenon on the Waikato River has been solved - and no, it's not a volcano.
A spokeswoman from the Waikato District Council confirmed tonight that the cause was in fact "backwash'' from a nearby water treatment plant.
In a statement sent to the Herald tonight, the spokeswoman said: "It's the water treatment plant intake screen - the filter is being backwashed. It's standard practice and a regular occurrence when cleaning the pipes, so nothing unusual or untoward.''
Ngaruawahia resident Martin Smith came across the strange sight as he was travelling to work yesterday.
He initially thought someone on a jetski had passed by, but then saw the water bubbling again.