In 1991, workers in New York City made a troubling discovery. They found human bones. After examinations by experts, the remains were found to be of African-Americans, some of whom had been slaves. They had been buried in the 17th and 18th centuries. Now, members of Congress from the state of New York want a museum and education center built. They want to help people remember and learn about a very dark period in American history. The congressional members estimate that 20,000 people -- men, women and children -- were buried at the place in the lower part of the New York City borough of Manhattan. After the discovery of the remains, the burial ground was declared a National Historic Landmark in 1993. A monument was created in 2006 and later a visitor’s center opened. But supporters of the bill to build a museum and education center say more needs to be done so Americans will never forget about slavery. It was a period when people were shipped from Africa against their will, bought and sold, and made to work without pay. Many were separated from their families. “Their contributions to America in the face of great injustice and persecution should not be forgotten,” said Senator Kirsten Gillibrand, a Democrat from New York.