Very little about the conception and design of the National September 11 Memorial Museum has followed the traditional narrative associated with the development of signature exhibition spaces.

The museum, which will be dedicated on Thursday and opens to the public on May 21, is the product of years of public outreach and often wrenching debate. Its exhibition spaces reflect a carefully considered effort to tell the story of 9/11 to an unusually broad audience, one that includes everyone from survivors and relatives of the nearly 3,000 victims to foreign tourists and children with no firsthand recollection of the attacks.

And then there is the space itself, a cavernous, nearly 124,000-square-foot subterranean expanse with imposing physical constraints—most prominently the undersides of the memorial pools that occupy the footprints of the original World Trade Center towers, and tracks carrying tens of thousands of daily commuters to and from the adjacent PATH rail station.

The challenge of transforming this unorthodox space into a setting for education, reflection, and solemn contemplation fell to a team of architects from the firm Davis Brody Bond and exhibition designers from Thinc Design and Local Projects, all of which are based in Manhattan. (A separate team of architects from Snøhetta, which also has an office in the city, designed the aboveground steel-and-glass pavilion where visitors will enter the museum from the memorial plaza. A second floor houses an auditorium, a small café, and a pair of private rooms reserved for victims’ relatives.)

“If you think about what we had as a site, and that a traditional museum is an icon containing exhibits, this is the inverse. The exhibit is the icon,” Steven M. Davis, a partner at Davis Brody Bond, told writers and editors who gathered at the museum for a media tour on Monday. “So we have the memorial and pools as as if impressed on the site, we have the slurry wall to the east, we have the PATH mezzanine. Our roof is the memorial plaza. So the only thing that we really imposed as architects on the resources that were to become our site is this thing that we call the ribbon.”

The ribbon is a gently sloping walkway that begins after visitors descend a staircase from street level—passing a pair of 80-foot-tall steel columns from the North Tower—and gradually guides them from a concourse level down to bedrock, some 70 feet below the plaza. It is surfaced in dark-brown walnut, a material that Carl F. Krebs, a partner at Davis Brody Bond, said was chosen to signal that “you’re beginning to make a transition into something that’s a bit quieter and more contemplative” than either the memorial plaza or the light-filled pavilion above.

“We felt we needed to give visitors a lot of time and space to pace themselves,” Krebs said. “If you think of other museums, you go into a lobby, you get your ticket and go into an exhibit immediately. What you’re going to see now is a long procession before you ever reach something like a curated exhibit. So the idea of these large spaces, these attenuated spaces, was really a conscious decision to pace the visitor and bring them into this world that’s going to have some very challenging emotional content.”

The ribbon gradually narrows as it reaches a large-scale photograph of the Twin Towers taken at 8:30 a.m. on September 11, 2001, 16 minutes before American Airlines Flight 11 crashed into the North Tower. Here, ordinary people from around the world share their 9/11 experiences in short audio clips, their words flashing across a series of screens that, when viewed from a distance, form a map of the word. Projected images of the handmade posters that distraught relatives made for their missing loved ones are projected on a wall.

Immediately beyond this is an overlook where visitors can gaze out at Foundation Hall, getting their first glimpse of a 60-foot section of the slurry wall—part of the original Trade Center foundation that held back the Hudson River after the attacks—and the shells of the memorial pools. These volumes, which stand in for the absent towers, are sheathed in aluminum panels that have been injected with gas, giving them a textured, reflective surface. They appear to float above the rusted outlines, embedded in the floors, of the box columns from the original towers. “Those were identified as historic resources and we were required to preserve them,” said Mark Wagner, an associate partner at Davis Brody Bond. “By preserving those columns, which basically created these two one-acre footprints, and the slurry wall, it established the scale of this museum.”

From here, the ribbon carries visitors down a staircase, past a preserved section of the so-called Survivors’ Stairway, and deposits them at bedrock, facing a repository where some 14,000 unidentified human remains from the attacks have been entombed, over the objections of some relatives. The repository’s concrete wall bears a quote from Virgil’s “Aeneid” (“No day shall erase you from the memory of time”), forged from Trade Center steel, that itself proved controversial when scholars suggested it had been taken out of context.

Some larger artifacts—a section of the Trade Center’s radio and television antenna; a fire truck, its cab a tangle of steel—are on display in the large “interstitial” spaces, as the Davis Brody Bond team calls them, at bedrock level. But visitors must cross a threshold, or “bridge,” into the tower footprints themselves to access the museum’s historical collection and its tribute to the victims.

The North Tower footprint houses a permanent exhibit that tells the story of 9/11 in photographs, video, and audio recordings, and through artifacts recovered from the rubble. Some of the more difficult material—images of people falling from the doomed buildings, for instance—is tucked into alcoves beyond signs warning of its sensitive nature, giving visitors the choice of whether to engage with it.

“You can experience this place as you choose,” said Tom Hennes, principal at Thinc Design, the lead exhibition designer for the museum. “For those for whom it would be overwhelming to see the most difficult things, you can stay well away from that because they’re well signed, they’re well signaled. The day is quite distant, so using time as a separator helps move people through and helps them choose how to engage. We can’t please everybody, but that doesn’t mean that we choose a slippery medium that doesn’t accomplish anything. What we’ve tried to do is understand that people have different goals and people have different needs, and we address those in different ways in different places. We don’t try to do everything at the same time.”

The South Tower footprint contains portraits of the 2,983 people who were killed on 9/11 and in the 1993 World Trade Center bombing, and touch screens where visitors can access snapshots, information, and recorded reminiscences for each of them. “It’s husbands telling stories about wives, or best friends who were at each other’s weddings,” said Jake Barton, principal at Local Projects, which designed the interactive exhibits for the museum. “I think in some way it’s some of the most emotional material just because it’s so personal. You have these huge pieces of steel and you see a fire truck and it seems so unreal, and then you hear stories about how someone was an amazing Little League coach and it crushes you because it’s just so familiar.”

Both exhibition spaces were off limits to the media on Monday’s tour; relatives and other “stakeholders” (survivors and first-responders among them) will have a chance to view them first over a six-day period starting Thursday when the museum will be open around the clock exclusively to them.

Part of the challenge for the exhibition designers was to document an event that has yet to fully recede into history. “There is no established 9/11 history,” Hennes said. “We’re still living the outcome of this event, so there’s no settled narrative. So the best way we felt to present it was through a variety of perspectives, to let people go through and to trust them to position themselves within that narrative.”

Barton noted that “Timescape,” a series of 9/11-related timelines that will be projected on one wall of Foundation Hall, will draw from an archive of news articles dating to September 11, 2001—an archive that will continue to grow as time moves on. And visitors will have the opportunity to record their own 9/11 experiences, he said, or add to the collection of testimonials about someone they knew who died. “Anybody can come in and tell a story or add a story,” Barton said. “It really makes this place a museum that’s ever collecting and ever changing, and transforming as more visitors go through.”