In 1785, as an unknown African slave built the furnace for a plantation's greenhouse, he packed in this prehistoric pestle among the bricks. The object is a West African spirit practice symbol, University of Maryland archaeologist Mark Leone said in a prepared statement. University of Maryland archaeologists are excavating the grounds of the Wye House outside of Annapolis, Maryland. The house is known for its beauty—and for being a plantation where abolitionist Frederick Douglass was enslaved as a boy in the 1820s. The house and garden, where the greenhouse stands, appear in Douglass's autobiography: "Colonel Lloyd kept a large and finely cultivated garden, which afforded almost constant employment for four men," he wrote. "To describe the wealth of Colonel Lloyd would be almost equal to describing the riches of Job."

Now, urban archaeologists are unearthing the possessions of the slaves whose labors created that fabulous wealth. The researchers announced their greenhouse findings on February 14. In the same room as the West African pestle, they found pottery, coins and religious charms, which indicate that slaves lived in the greenhouse. The slaves constantly tended the plants and the complex furnace system, which warmed the greenhouse from under its floor.

—Francie Diep