A £270,000 project to preserve historic mine buildings on the outskirts of Redruth is being hampered by vandalism.
Work began in February at the Taylor's and Davey's sites in the Wheal Maid Valley at Crofthandy, part of what was once known as "the richest square mile in the old world".
But while work has continued at pace during weekdays, staff working for contractors, Darrock and Brown Ltd, of Bodmin, have returned to work after weekends to find a trail of wanton destruction.
The door of a metal rest room was forced open and the interior trashed on one occasion.
On another, bolt cutters were used to cut padlocks on a metal storage container.
A metal gate at the entrance to the site was cut from its hinges and stolen.
Last weekend a portable toilet was cut open and vandalised and bags of sand used for pointing were tipped over, and scaffolding poles dropped down a capped mine shaft.
And on Monday night, vandals struck again, cutting a chain at the entrance to the site and breaking the lock on a container only to find it empty.
Bill Gladwell, from Darrock and Brown, said: "Somehow they managed to break in through the steel door of the first accommodation block we put up there and then trashed it.
"We had to take it away and replace it. It costs £100 a week to rent, plus haulage to get it there. We waiting for an invoice for that one.
"We have a £1,000 excess on our insurance.
"They've tried to steel a generator and a cement mixer, and several weeks ago they broke in and petrol.
"They threw scaffolding poles down a shaft. It's just wanton vandalism.
"It's just frustrating to know that because we are based in Bodmin we can't keep an eye on the site all the time."
Over the years Darrock and Brown Ltd has worked on 46 churches, 24 engine houses and Truro Cathedral.
"This is the first time we've ever had problems while working on an engine house complex," added Mr Gladwell.
CCTV images are being reviewed and all the incidents have been reported to the police. Anyone who witnesses anything suspicious is urged to report it on 101.
