Most New Yorkers don't even know it's there. Hart Island, near the popular summer spot City Island, is one of the world's largest cemeteries, and the U.S.'s largest potter's field, where the indigent and unidentifiable have been buried en masse since just after the Civil War. Inmates from Riker's Island perform the burials, at a rate of some 2,000 per week; there are more than 800,000 buried here altogether, three-deep in cheap wooden boxes, in long trenches that cover more than half the island. At one time the island also housed a prison, a boys' workhouse, a Nike Ajax nuclear missile silo, and for four months in 1865, it was a prisoner of war camp used to house captured Confederate Troops, more than 250 of whom died and were buried here. The only grave with a marker is that of an unnamed baby who died in 1980, New York City's first AIDS casualty, buried in isolation.
Unless you have a relative buried on the island, you can't go there, so very few people ever get to see it for themselves. In 1978, a news crew visited the island for a day, and came away with a nice piece and some great footage of some buildings that don't exist anymore, and of prisoners on burial detail -- check out the way they throw the infants' coffins to one another.
Photo by Joel Sternfeld, from his book about the island.