DAVENPORT, Iowa (AP) — A Davenport cemetery has been listed on a national U.S. National Park Service registry of sites connected with the Underground Railroad, an anti-slavery network that had helped many escape to freedom.

A high school social studies class from Arlington Senior High School in eastern Nebraska discovered the graves of 11 former slaves at Oakdale Memorial Gardens in Davenport through a research project.

Their teacher, Barry Jurgensen, says finding so many ex-slaves buried in Oakdale was one of the greatest discoveries he’s ever made.

The class traveled to Davenport in March to research the story of Milton Howard who, as a child, was kidnapped in Muscatine County and sold into slavery. He eventually escaped, joined the Union Army and settled in Davenport.