The concrete shell in downtown Boston that will soon be the world’s largest Passive House office building is easy to miss, a nondescript construction site across from the city’s “Pregnant Building.”

The Millennium Partners project now sits in the shadow of the developer’s most recent build, Millennium Tower. But Winthrop Center, which at 691 feet would become Boston’s fourth-tallest building, is expected to stand out as one of the world’s greenest buildings when it opens, which is expected to be late next year.

The tower has risen to 12 stories as of last week when Bisnow toured the construction site, near where Millennium Partners is meeting its first prospective office tenants at a high-tech leasing center, which it has dubbed the "Discovery Center."

“You put your company in a building like this, you’re going to retain the best and recruit the best talent that’s out there,” Millennium Partners partner Rich Baumert said in an interview from a leather chair in front of the leasing center’s “three-sided surround screen,” akin to a mini-IMAX movie theater.

“Because everybody, there is no question, particularly during the last 12 months, we've all felt so helpless,” Baumert said. “This is something you can do to really make a positive contribution.”

Prospective commercial tenants visiting Millennium Partners' Discovery Center will find stylish meeting rooms, 3D-glass printed light fixtures in the shape of Winthrop Center’s logo and the screening room behind massive sliding doors. The presentation is important to conveying the lifestyle Winthrop Center has to offer, Baumert said.

“From the moment you walk through the front door, it's everything, it's the candles that are lit, it's the music that plays. That really is all by design,” Baumert said. “It's all choreographed, it's very important for us to make sure that people understand that we really are on every detail.”

The presentation opens with sweeping shots of Boston, narration highlighting the city and Millennium Partners' innovative nature, and booming music. The show presents clips of project designers touting the building’s eco-friendly bona fides.

Baumert, between scenes, shared the story of how Winthrop Center’s design was inspired, beginning with a trip to The Edge Amsterdam, a 40K-square-meter office building deemed in 2015 the greenest in the world. Inspired, the developers reached out to the minds at the Massachusetts Institute of Technology and got in touch with professor John Fernandez, who was instantly sold on helping out, Baumert said.

Fernandez, the head of MIT’s Urban Metabolism Group, took a look at Winthrop Center’s plans and wrote a report for the developers, which now sits in Discovery Center’s library.

“Ever since we said that we reached out to him, his phone has been ringing,” Baumert said.

As of mid-March, the most visible feature of the development is the massive Connector, a walkway which will feature eateries, arts installments and views to the building’s office floors. A sense of its scale can be drawn from the sidewalk when looking at the crane under The Connector’s roof, holding a massive clamshell bucket used to scoop up the soil.

Not visible to passersby and yards beyond the site’s fence, sectioned-off portions of the floor stretched four stories into the ground, which allow concurrent work in the basement and above.

Millennium Partners principal Kathleen MacNeil led a hard-hat tour of the Suffolk-led construction site on a late afternoon after most site workers had left. Speaking over the periodic banging of concrete being pumped through pipes to the building’s 12th floor, MacNeil described the “up-down construction” method, which allowed for construction to continue during winter weather.

“It opens up a lot of areas of the building for other trades to work,” MacNeil said. “It’s actually been around for a long time, but not many people do it. It’s more expensive.”

The T-shaped floor plates cut in just yards away from neighboring office buildings. Floor plates spanning 38K SF will soon don massive 5-inch-thick, 40-foot-long pieces of glass similar to those used at Apple’s headquarters.

The building, designed by Handel Architects, is expected to meet Passive House specifications for its office portion in line with Boston's carbon neutrality goals. Technologies such as MERV-15 air filtration, touchless entry points and temperature controls are easier for tenants to understand in a post-Covid world, Baumert said. Millennium Partners' presentation, in its advertising of meeting WELL Gold, LEED Platinum and Passive House building certifications, makes no mention of the pandemic.

“When we first started talking about this, it was tough for people to sort of get their arms around,” Baumert said. “It gives us an opportunity to have an easier discussion with people who really realize this is now where the future is going, but where everyone needs to get to today.”

CORRECTION, MARCH 29, 11 A.M. ET: A previous version of this story incorrectly identified the building as reaching a zero operating carbon footprint and misidentified which portion of Winthrop Center would be Passive House certified. The story has been updated.