BODMIN-based contractors say persistent vandalism is preventing the restoration of historic mining buildings.
A £270,000 project at the Taylor's and Davey's site in the Wheal Maid Valley at Crofthandy, near Redruth – part of what was once known as "the richest square mile in the old world" – began in February.
But workers for contractors Darrock and Brown Ltd, of Bodmin, have returned to the site after weekends to find a trail of 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 site entrance was cut from its hinges and stolen.
A portable toilet was also cut open and vandalised, bags of sand used for pointing were tipped over, and scaffolding poles dropped down a capped mine shaft.
Vandals also cut a chain at the entrance and broke the lock on an empty container.
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 are still waiting for an invoice for that one.
"We have a £1,000 excess on our insurance.
"They've also tried to steal a generator and a cement mixer, and several weeks ago they broke in and took a quantity of petrol.
"They threw scaffolding poles down a shaft. It's just wanton vandalism.
"It's just so frustrating to know that it's happening because we are based in Bodmin and we can't keep an eye on the site all the time."
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 anything suspicious should be reported to the police on 101.
