From BBC News:
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The remains of 20,000 African men, women and children have lain beneath the busy streets of New York for 300 years, waiting to tell their stories on the extent of slavery in the city.
In March 1992, leading African-America archaeologist Michael Blakey arrived at the burial ground in downtown Manhattan.
"I had read about these people documented as chattel, " he said. "Now I was going to learn about these Africans in New York as human beings."
A haunting sight greeted him. Being winter, work was taking place under a translucent plastic tent. "I'd really never seen an excavation like that one," he said. "There were mini excavators working and kerosene heaters going."
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More:
http://news.bbc.co.uk/1/hi/world/americas/3659397.stm