Illegal metal detecting may mean that the full story of the Roman occupation of the Marton site may never be known.
Police are investigating after illicit 'nighthawkers' gained access to the dig on Saturday, September 22.
"There were definitely nighthawkers digging without permission and they made quite a mess, with holes all over the place," said Lincolnshire County Council's historic environment officer Sarah Grundy.
"It's a great shame because genuine metal detecting enthusiasts have already been very helpful during walks over the site with landowners' permission.
"And, although all the historical finds had been safely removed by the team when this incident took place at the weekend, we'll never know if these people did find anything and take it away."
Witnesses or anyone with information about the illegal metal detecting can call the Lincolnshire Police non-emergency number on 101 or Crimestoppers anonymously on 0800 555111.