Newly unsealed documents show that federal workers worked at a remote site in Pennsylvania to uncover what could be nine tons of gold stolen from the US Mint by Confederate sympathizers during the Civil War, the Associated Press reports.

An FBI agent from the bureau’s art and antiquities theft division filed an affidavit in 2018 with the federal court claiming the FBI had a reason to believe an unspecified amount of gold was buried in a cave in Dent’s Run in the north central part of the state.  The FBI filed the affidavit to prevent the state’s Department of Conservation and Natural Resources from staking a claim on anything that might be found.

The FBI based their affidavit on information gleaned from a father-and-son team of treasure hunters.  The information was so credible the FBI brought in a contractor with a ground sonar system that can identify the size and density of underground objects.  The contractor identified an area consistent with the size and density of nine tons of gold.

According to the FBI, however, the site revealed nothing.

According to lore, the gold was being transported to Philadelphia when it was hijacked by a group of Southern sympathizers called the Knights of the Golden Circle.  Reportedly, the cave the FBI has been searching had a carving of a turtle at the entrance, believed to be a sign of the Knights of the Golden Circle.

Nine tons of gold in today’s market would be worth more than half a billion dollars.