4. The Hidden Survey Bolt
The bolt is believed to be one of the original survey bolts from when the Manhattan grid was first planned in 1811. Led by John Randel, surveyors had the task of marking each future street intersection aligned with the grid. They used marble monuments or iron bolts where there was exposed bedrock. As Central Park was not a part of the original plan for Manhattan, street intersection points may have been marked but left to remain since no intersection was actually built here.
The bolt is known to be in the park, but the exact location is kept a secret for fear that it will be stolen or destroyed.