By Molly Daly

PHILADELPHIA (CBS) – A special gathering was held in Old City Sunday to call attention to the plight of the women and girls trapped in the dark world of sex trafficking.

This was part of the Red Sand Project — an exercise in activism through public art.

Participants gathered on 3rd Street above Arch, filling cracks in the sidewalk in front of the Old City Jewish Art Center with red sand.

“The red sand symbolizes filling those cracks, so that vulnerable people don’t fall between them any longer,” explains Carol Metzger, who is the author of Facing the Monster: How One Person Can Fight Child Slavery.

Participants are urged to post pictures of their sidewalk transformation on social media with the hashtag #RedSandProject — an act that may seem inconsequential, but which can raise awareness of the scourge of human trafficking.

And it’s not just in some far-off land.

“What people don’t understand sometimes is that it’s here,” says Metzger. “It is right here in Philadelphia, in Southeastern Pennsylvania. It’s a problem we all need to face.”