1. The Old City Hall Subway Stop
2. The Freedom Tunnel, Riverside Park Tunnel
3. Part of the Berlin Wall, 520 Madison Ave.
4. Network of Tunnels Beneath Columbia University
General info: Hidden beneath Columbia University are a series of tunnels and rooms connecting the entirety of the campuses' buildings. The oldest tunnels apparently existed from the days when Columbia used to be a mental asylum.
Accessibility: This is technically closed to the public. It is easier to get in with a Columbia ID, but it is still illegal.