Now in its seventh year, Open House offers 150 cultural events and takes place weekend, October 10 and 11. Organizers say it is the nation's larges architect and design event.
Many of the sites, which include places like Newtown Creek Wastewater Treatment Plant and several private residences, are otherwise closed to the public.
This year's sites include 7 World Trade Center, the Alice Austen House Museum, the Apollo Theater, the Atlantic Avenue Tunnel --thought to be the world's oldest subway tunnel -- and the newly constructed Standard Hotel.
The tour guides are often intimately connected to the sites. Owners give tours of their private residences and Hugh Hardy, restorer of Radio City Music Hall, leads groups through the iconic venue.
The program runs with the help of 600 volunteers and has grown in popularity since its start. Some sites require visitors to sign up ahead of time -- but the only other requirement to take part is plain old curiosity.