Criminals have struck at a Highland dam, removing 18 “large” stones from the base of the facility.
Police have described the incident at the hydro dam at Lairg in Sutherland, as “vandalism”.
They have not revealed details about the size of the stones involved, but suggested some sort of equipment would be required to move them.
It is understood that power firm Scottish and Southern Energy alerted the police on Wednesday – although the stones were dislodged some days beforehand.
It is not clear at this stage if the incident has disrupted power production.
A spokesman for the force said: “The stones are thought to have been deliberately dislodged from the base of the dam and placed in the water.
“As a result of damage sustained, the dam will require a costly repair.”
He added: “It is believed the stones were removed over a period of time from May 10 to May 18.”
Police want to hear from anyone with information on 101 or Crimestoppers on 0800 555 111.