Lower Manhattan was greatly extended by landfill, and now tourists may learn about much of New York City's early history by visiting these Lower Manhattan sites. During the construction of a new federal office building in 1991, the buried bones of around 400 Africans from the city's colonial past were discovered here. The remains dated back to 1677, when the first slave trade license was issued by the colony of New York. Some of the oldest buildings in America are located in Lower Manhattan. A few of them remain today: the First Presbyterian Church, built in 1735; the Second Presbyterian Church, built in 1872; and the Fifth Avenue Methodist Church, built in 1892. These churches show the growing importance of education in New York after its state constitution was revised in 1777 to guarantee religious freedom. The site where the World Trade Center stood before it collapsed in September 2001 is covered with construction debris. But inside this rubble are evidence of what went on above ground during the last days of the towers. Rescue workers found documents, phones, computers, and files that provide insight into how people planned for their deaths in the event of an airplane crash. Also in 2001, archaeologists made a surprising discovery while excavating part of the site of the former United States Mint. They found a cannon ball that had been shot from the deck of the USS Constitution, the largest ship in the American navy.