Malvern, Pennsylvania (CNN) -- "This is a mass grave," Bill Watson said as he led the way through the thick Pennsylvania woods in a suburb about 30 miles from Philadelphia.

"Duffy's Cut," as it's now called, is a short walk from a suburban cul-de-sac in Malvern, an affluent town off the fabled Main Line. Twin brothers Bill and Frank Watson believe 57 Irish immigrants met violent deaths there after a cholera epidemic struck in 1832.

They suspect foul play.

"This is a murder mystery from 178 years ago, and it's finally coming to the light of day," Frank Watson said.

The brothers first heard about Duffy's Cut from their grandfather, a railroad worker, who told the ghost story to his family every Thanksgiving. According to local legend, memorialized in a file kept by the Pennsylvania Railroad, a man walking home from a tavern reported seeing blue and green ghosts dancing in the mist on a warm September night in 1909.

"I saw with my own eyes, the ghosts of the Irishmen who died with the cholera a month ago, a-dancing around the big trench where they were buried; it's true, mister, it was awful," the documents quote the unnamed man as saying. "Why, they looked as if they were a kind of green and blue fire and they were a-hopping and bobbing on their graves... I had heard the Irishmen were haunting the place because they were buried without the benefit of clergy."

When Frank inherited the file of his grandfather's old railroad papers, the brothers began to believe the ghost stories were real. They suspected that the files contained clues to the location of a mass grave.

"One of the pieces of correspondence in this file told us 'X marks the spot,'" said Frank. He added that the document suggested that the men "were buried where they were making the fill, which is the original railroad bridge."

In 2002, the brothers began digging and searching. They found forks and remnants of a shanty and, in 2005, what Bill Watson calls the "Holy Grail" -- a pipe with an Irish flag on it.

They knew they were close, but Bill said they knew they needed "hard science" to get them to the next step.

The science came from Tim Bechtel, a geophysicist, who learned about the project from a colleague at the University of Pennsylvania who had heard the Watson brothers speak. The friend knew Bechtel could provide the missing link in the brothers' excavation efforts.

Bechtel's work included earth scans, which can help detect what's underground without digging or drilling.

By shooting electrical current through the slope, Bechtel said he learned there were "oddball areas" or places where the current wouldn't pass through. "We saw areas in the slope that were very electrically resistant," Bechtel recalled.

This was an initial indicator something might lie beneath the surface. After further digging, Bechtel and the Watsons detected "air bubbles above the coffins," he said.

Bechtel helped pinpoint key areas to dig and on March 20, 2009, Bill Watson said the team made a startling discovery.

"One of my students came running over at about 2 in the afternoon with something that was a clearly discernable human bone," Bechtel said.

It was just the beginning of the many puzzle pieces to surface at Duffy's Cut. The pieces led them to suspect that something other than cholera was responsible for the deaths.

"A teeny weenie little fragment like that is so chock full of information," said Janet Monge, holding up a jawbone and teeth found at the Duffy's Cut site. She believes the teeth, because of their irregularities, could someday be linked through DNA to living descendents of the men unearthed at the dig site.

Monge, an anthropologist at the University of Pennsylvania, joined the forensics team when Bechtel looked her up in the campus directory and asked for help separating the human bones from any animal bones.

Since then, Monge has collected bones from seven skeletons unearthed at Duffy's Cut, including four skulls. The trays and containers of bones occupy a long, wide table in the back of a lecture room at the University of Pennsylvania Museum in Philadelphia.

Poring over the bones with her green spectacles sitting low on her nose, Monge said she has focused her attention on the skulls, adding that they have provided crucial clues to what might have killed the Irishmen at Duffy's Cut.

"This skull has a little divot on what would have been the side bone of the skull," she said, holding it up. "That little divot is something that didn't happen when they excavated it out of the ground."

With just one divot on one skull, she was reluctant to jump to conclusions. But as more skulls surfaced, a pattern started to form. Holding the second skull, Monge said with confidence: "This person was clunked on the head at around the time of death."

Two weeks ago, a new piece of evidence came up from the ground at Duffy's Cut: A skull with a perforation that could be a bullet hole. "In fact, we can see some nice cracked edges that do look very much like a bullet hole," Monge observed.

Monge and the team will soon test the skull for the presence of lead. The source could be a bullet or an ax. Either way, she said, "If they had cholera, it didn't kill them. I would say something else killed them, but they might have had cholera, too."

Why is the mystery so important to the team?

"It could have been us," Bill Watson said. "These guys came over here with nothing, looking for the American dream like countless people have done. They thought they were going to make it and within six weeks of arrival they're literally buried in the fill here."

Although they have unearthed seven individual's remains, the Duffy's Cut team labors on to find the 50 more they believe are still underneath the surface.

The brothers said their goal is to preserve the memory of the Irish workers and to put the story in textbooks, to be remembered for years to come.

"It's a story that transcends nations, transcends history in a sense. It's the story you hear of workers that were exploited anywhere in the world," Frank Watson said.

"How do we treat our employees? How do we treat people who immigrate for a new life? Every human being deserves to be remembered."