So-called "off-road vandals" have been blamed for wrecking a £500,000 project to improve access to part of the County Durham coastline.
They have caused damage valued at thousands of pounds to a new stairway at Blast Beach at Nose's Point, Dawdon.
Work to restore access to the area, which is a Site of Special Scientific Interest (SSSI), had just finished.
Durham County Council said feature boulders had been ripped from settings and steps damaged by off-road vehicles.
Much of the cash for the project came from the Heritage Lottery Fund.
The area, near the former Dawdon Colliery and Seaham Harbour Railhead, offers spectacular views northwards to the Port of Seaham and south across Blast Beach, making it one of the most dramatic viewing points on the coast.
The scheme, which was to open for the summer season, will now be delayed while the boulders and steps are repaired.
The full extent of the damage and remedial works is still to be assessed.
Heritage coast officer Niall Benson said: "This is absolutely senseless, criminal vandalism by those who think they have the right to drive where they want, when they want, with absolutely no concern for anyone else but themselves.
"People out at the weekend must have seen something and if anyone has any information at all, please let the police know."
Durham Police are investigating.