The Ludlow Massacre more than a century ago was one of the most violent events in U.S. labor history and a wake-up call for the nation about brutal and often deadly coal mine work.

Recent preservation efforts at the site about an hour south of Pueblo have revealed symbols hidden for around a hundred years.

In 1914, southern Colorado coal miners were on strike to fight for safe working conditions and fair wages. On the day after their Greek Easter celebration, bullets flew through the sprawling United Mine Workers Ludlow tent colony. To escape the violence, four women and 11 children climbed into a hole dug in the ground beneath a tent. Then company thugs torched the encampment.

“My grandmother and another woman were the only two people who walked out of that pit alive,” Mary Elaine Petrucci told KRCC in 2014. She said the others suffocated in the dirt cellar as the tent colony burned.