CNN:

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."


Two investigators have recently proved that these ghosts were dancing on a mass grave.

"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.
"They were a-hopping and bobbing on their graves."