Pittsburgh, 1977. It’s Friday, December 23, and all along Liberty Avenue, Downtown office workers shop for last-minute gifts and drink yuletide cocktails on the street. Amid the holiday merrymaking, a cab arrives at Gemini Spa — one of the many “massage parlors” that line Pittsburgh’s red-light district. The driver hands a Christmas present to a spa employee, who rushes upstairs to open it.

That’s when the bomb explodes.

The explosion rips through the parlor, blowing out the building’s windows and raining glass on Liberty Avenue, according to eyewitness accounts published by the Post-Gazette that year. Outside, shoppers stop to look for a moment, then continue on with their day. They’re used to violence on Downtown’s seediest street. “Probably just another mob, massage parlor thing,” one woman tells a reporter as she walks away with her gifts.

Of course, scenes like this are long gone. The massage parlors have disappeared, and what was once Pittsburgh’s red-light district is now its renowned Cultural District. But on October 7 and 8, Pittsburghers can get an up-close look at the city’s past, present and future — including a tour of what used to be the red-light district — when Doors Open Pittsburgh returns for a second year.

The annual two-day event allows the public to experience Pittsburgh’s most iconic landmarks and cutting-edge spaces firsthand, says Bonnie Baxter, the event’s founder and executive director. After drawing more than 4,000 people Downtown in 2016, Baxter expanded the event this year to include the Northside and the Strip District. “We want you to be nebby!” she says. “Doors Open Pittsburgh gives you a reason to explore buildings that you may never have been in or heard about.”

This year, in addition to nearly 70 private offices, clubs, places of worship and theaters, open buildings include the historic Dollar Bank headquarters on Fourth Avenue, the Allegheny County Courthouse and Nova Place. Visitors can browse the full list and create their own itineraries on the event’s website, sorting their options by everything from whether there’s art on display to whether the building contains a vault. (“Pittsburghers go nuts over vaults,” says Baxter, laughing.) There’s even a search category for “amazing views” — something of which Downtown has no shortage.

A self-described “house-and-building junkie,” Baxter hopes to help natives and newcomers alike get to know the city in unexpected ways. “Pittsburghers and people who come here are always surprised by what they see in terms of architecture,” she says. “We have such a diverse collection of styles, and a lot of our buildings are very well maintained. It’s impossible to walk past some of them and not be a little curious. What’s inside? What’s this building’s story?”

Doors Open Pittsburgh gives attendees several ways to find out. Ticketholders can simply show up and explore buildings at their leisure, or they can register for “Insider Tours” — semi- or fully-guided experiences that encompass multiple buildings and themes. Focused on everything from food to the riverfront to the architect Daniel Burnham (of “The Devil in the White City” fame), Insider Tours are led by experts and storytellers from around the region.

Among them will be Christopher Whitlatch, CEO of the Mon Valley Alliance and creator of the “Taking Liberty” tour, which explores the bygone red-light district. Inspired by newspaper stories from the 1970s that “read like the screenplay for ‘Goodfellas,’” Whitlatch developed the tour to remind Pittsburghers of a grittier, grimier time. “Those kinds of stories aren’t told very often anymore,” he says. “These days, we like to talk about how great Pittsburgh is. And that’s wonderful — I love that. But Pittsburgh’s underside is interesting, too, and I think learning about it helps us truly appreciate just how much the city has transformed.”

Tours like Whitlatch’s “are part of what makes the experience so cool,” says Baxter. “We’re going to give people something a little different, going above and beyond the buildings themselves. Most of us forget things like what a style of architecture is called or when certain buildings were built, but everybody appreciates a good story. And that’s what we hope people will take away from Doors Open Pittsburgh — lots of good stories.”