In August of 1862, Union forces faced off against the Rebels near Manassas, Virginia, in the Second Battle of Bull Run. It was a doomed struggle, larger in size and, ultimately, casualties than the First Battle of Bull Run, and ended in a hasty Union retreat, with soldiers’ bloody bodies scattered across the battlefield.

The remains of some of these soldiers were lost forever, unceremoniously buried far from their families. That was the seemingly the fate of two Union men who died of gunshot wounds and were buried in a shallow ditch with a collection of amputated limbs from their fellow soldiers. But in late 2015, a worker at the Manassas National Battlefield Park made a fortuitous discovery, unearthing bone fragments while clearing land for a pipeline. Sure enough, they were human.

National Park Service experts and anthropologists with the Smithsonian’s National Museum of Natural History teamed up to scout out the area, eventually realizing they’d stumbled on something far greater than bone fragments: a “limb burial pit” with the remains of two soldiers and 11 limbs that had been amputated after the Second Battle of Bull Run.

It’s the “first time in history” that a Civil War surgeon’s pit has been excavated and studied by experts, according to the National Parks Service.

“The harrowing stories of Civil War soldiers and the surgeons who tended to them are traced in the remains and bones of these men,” Smithsonian anthropologist Douglas Owsley said. “We’re humbled by the opportunity to give voice to their story, and we are in awe of the bravery and tenacity these men showed in the face of war.”

The soldiers, who were between 25 and 34 years old when they died, will be buried at Arlington National Cemetery later this year in coffins fashioned out of a tree from the battlefield. One of the soldiers was found with a bullet from an Enfield rifle musket, a type of gun used by Confederate soldiers, still lodged in his femur. The bullet offered a powerful clue as to how the soldier was taken down in battle.

“He’s retreating, withdrawing,” Owsley told the Smithsonian magazine. “He’s shot in his buttocks area, really high.”

It was also one of many clues that led the anthropologists to determine that the bones belonged to Union soldiers. They discovered buttons from a Union jacket on the second soldier, who was killed by three fired lead buckshots. Even more incredibly, Owsley and his team performed isotope analysis on the remains that determined the soldiers had consumed food and water from the North as their bones formed.

“Oxygen isotopes tell us about their drinking water,” anthropologist Kari Bruwelheide told Smithsonian. “And that varies by region, so we were able to place these men in the northern states.”

Experts believe it’s likely that a field surgeon deemed the soldiers’ wounds too grave to operate on. The other amputated bones discovered in the pit offer insight into medical practices at Civil War field hospitals. Amputations would have been hasty—sometimes taking less than 10 minutes—and with surgeons using the most rudimentary of tools, according to the Smithsonian. But, in this case, they were performed by an experienced surgeon.

“These soldiers and the amputated limbs deepen our understanding of the techniques field surgeons used to save the wounded in the midst of battle,” Manassas National Battlefield Park Superintendent Brandon Bies said in a statement. “The discovery also tells us about the difficult decisions doctors faced about who could be helped and who could not.”