Consider it the Super Bowl for architecture buffs.
Open House Chicago returns this weekend to give visitors from around the globe the rare opportunity to step inside some of the city's most treasured landmarks typically closed from public view.
The now-annual event run by the Chicago Architecture Foundation (CAF) provides a city-wide history lesson spanning 18 neighborhoods and 150 structures, CAF officials told The Huffington Post.
This year, though, a handful of landmarks on the tour will open to guests for the first time in more than half a century. Among the most anticipated is the Allerton Hotel -- home to the iconic "Tip Top Tap" sign -- which has been closed since 1961.
The hotel's bar and ballroom once hosted stars of the day like Bing Crosby, Bob Hope, Frank Sinatra and Lucille Ball, according to ABC Chicago. The ballroom is the featured stop inside the Allerton, which was purchased by a French hotelier earlier this year and renamed the Warwick Allerton Hotel.
You'll also get a peak inside downtown's The Sky-Line Club, one of the oldest private membership facilities in town. Nestled in the Old Republic Building, it was constructed in part in Sussex, England before making its way to the Windy City.
Not every stop on this year's open house tour is steeped in history, however. Among the two featured stops for 2014 are the headquarters of two millennial-era e-commerce giants: daily deals site, Groupon, and popular t-shirt maker, Threadless.
OHC organizers say they hope the free event draws at least 60,000 visitors, up from last year's crowd of 55,000. In 2013, visitors from 71 different countries and all 50 states poured into the city to take a peek into the best-kept secrets in Chicago.
Open House Chicago runs Saturday, Oct. 18 and Sunday, Oct. 19. Free, no tickets are required.
Caption information courtesy of the Chicago Architectural Foundation.