PUEBLO, Colo. (AP) — 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, KRCC-FM reports.

A tent colony of coal miners striking for safe working conditions and fair wages met with violence. Four women and 11 children climbed into a hole dug in the ground beneath a tent that was torched by company thugs.

Preservationists working in the cellar have found the shape of a shield, a cross and a tent stake that would have been holding down the tent covering the cellar where the women and children were killed.