Specialists using ground-penetrating radar in New York City have detected hundreds of previously undiscovered gravesites in an African-American burial ground that dates to the 1830s.

The New York Landmarks Conservancy announced the discovery Monday at Staten Island's Rossville A.M.E. Zion Church Cemetery, part of the historic Sandy Ground community that was among the nation's first free black settlements.

Before the radar survey there were 97 known burial sites on the 1.6-acre burial ground. Conservancy President Peg Breen says another 576 were discovered by radar at an average depth of 10 feet.

Sandy Ground was first settled by African-American oystermen. More than 150 families eventually moved there. The current Zion Church dates to 1897 and became a major stop on the Underground Railroad for escaped slaves.