Illegal metal detecting which damaged part of a Lincolnshire castle is being investigated by specialist officers.
Eight holes, between 1ft and 4ft square, were found dug in Bolingbroke Castle near Spilsby.
Police from Operation Totem, a crackdown on the illegal search of heritage sites known as nighthawking, have appealed for information.
Officers said such actions also damaged property and crops and often went hand in hand with other criminal activities.
Crime Prevention Officer Pc Nic Hanson said, "Nighthawking sounds like a very romantic activity, but those who indulge in it are not Indiana Jones or Lara Croft figures; they are thieves and those who steal our national treasures from underground are no different to those who steal the lead from our church roofs.
"Whilst there are many principled people who enjoy metal detecting as a perfectly legitimate hobby, it is becoming apparent that there are others who view it as a lucrative means to fund a drug and criminal lifestyle".
Heritage Lincolnshire, which manages the castle, said the area targeted did not contain any historically significant items.