The macabre Civil War relic is just off M Street SE, mounted on the Washington Navy Yard's Building 28. It's a black metal plate bearing this inscription: "WITHIN THIS WALL IS THE LEG OF COL. ULRIC DAHLGREN U.S.V. WOUNDED JULY 6TH 1863 WHILE SKIRMISHING IN THE STREETS OF HAGERSTOWN WITH THE REBELS AFTER THE BATTLE OF GETTYSBURGH."

The story is even stranger than it might seem.

Dahlgren was a 20-year-old Army captain leading a cavalry charge through Hagerstown, Md., when a Confederate round struck his right ankle and passed through his foot. Three days later he made it home to Washington, but fever set in, and military surgeons had to amputate his leg just below the knee.

Dahlgren's father, Rear Adm. John Dahlgren, had recently served as commandant at the Navy Yard, where a new gun foundry was being built. The day after the amputation, an ambulance took the leg to the yard, and a Marine honor guard escorted it to the foundry, where the building's cornerstone was about to be laid. The leg was placed in a box, draped with an American flag and sealed inside the cornerstone.

Despite the amputation, Dahlgren subsequently returned to battle. Promoted to colonel while recuperating, he left his sick bed to help lead a dar- ing but controversial and ill-fated 1864 raid on Richmond. His stump had not yet hardened enough to attach a wooden leg, so Dahlgren rode with his thigh lashed to the saddle, crutches strapped on the side of his horse. He was killed when Confederate troops ambushed his detachment.

Now comes the truly strange part. The foundry was torn down after World War II, but the leg was never found. The plaque nevertheless was moved to another building, where it remains today, a reminder of the sacrifices of war.