A rendering of what the Smithsonian’s southern campus would look like at night if construction plans proceed. (Courtesy Smithsonian Institution)

The Smithsonian unveiled a $2 billion plan Thursday that would reimagine the area surrounding its iconic Castle by creating new entrances and connecting underground galleries in what has been described as the biggest project considered for the Mall in more than a century.

Designed by architect Bjarke Ingels, the proposal calls for renovations to the red-stone Smithsonian administration building, known as the Castle, and the addition of two underground levels of visitor amenities, including a cafe, a store, an auditorium and restrooms. The new spaces would connect to the S. Dillon Ripley Center, the Arthur M. Sackler Gallery and the National Museum of African Art, which are all underground and, despite aboveground entrance pavilions, can be hard to find. The design would replace the pavilions with glass-walled entries visible from the Mall and rising from the corners of the Castle’s Enid A. Haupt Garden.

“We are blurring the boundaries between landscape and architecture,” said Ingels, founder of Bjarke Ingels Group. “We are creating an interesting relationship between what is aboveground and underground.”

In the works for two years, the idea is still a long way from fruition. The National Capital Planning Commission and the Commission of Fine Arts, the government agencies with design oversight, must approve the proposal for it to go forward. Construction is not expected to begin for five to seven years and could take as long as 20 years to complete.

The plan would extend from the Castle to the Freer Gallery of Art, the Arts and Industries Building and the Hirshhorn Museum and Sculpture Garden, creating a unified space that Ingels described as “150 years of architectural history of the United States.”

A rendering of the National Museum of African Art entrance from Independence Ave. (Courtesy Smithsonian Institution)

The formal Haupt garden — which is now the roof of the Sackler, the Ripley and the African Art Museum — would be replaced by a modern green space surrounded by skylights that would flood the galleries with natural light and, Ingels said, provide “sneak peeks” of the exhibitions below. Visitors could also see the green space from underground.

“We will create a more intuitive visitor experience to all the museums and orient the museums more toward the Mall,” Ingels said. “We want to increase the engagement with the garden, make it a more active component.”

No detailed budgets or timelines have been crafted, officials said, in another sign that there are many hurdles to overcome before a shovel hits dirt. The design would be financed by federal dollars and private donations, although Smithsonian officials could not provide specifics. Some of the projects are included in the recently announced $1.5 billion Smithsonian campaign, officials said.

Smithsonian Secretary Wayne Clough said the idea for the ambitious project emerged from conversations about long-overdue repairs and renovations to the buildings on the south side of the Mall.

“If we start renovating each building one at a time, we’d miss out on the opportunity to change the visitor experience,” Clough said. “We started to think less about the problems we face and more about the opportunities we have.”

Central to the plan is an overhaul of the Castle, which would include restoring the 1855 structure’s Grand Hall to its original size and protecting the building from earthquakes.

“This certainly could be the biggest project that has been proposed in the 110 years since the McMillan Plan, as far as the intricacies of the project and the price tag as well,” said Kirk Savage, a professor of history of art and architecture at the University of Pittsburgh. The 1902 MacMillan Plan created much of the Washington core of parks and memorials that we know today, including the Lincoln Memorial and the Mall.

The now-empty Arts and Industries Building would undergo renovations to house the proposed Smithsonian American Latino Museum, which awaits congressional action.

Farther east, the wall around the Hirshhorn would be removed and its circular fountain lowered by a level, bringing light into the lower galleries. The museum of modern and contemporary art also would gain gallery and public space.

As for the visitor experience, Smithsonian officials said they hoped the changes would make the area more welcoming in the after-hours.

But Savage isn’t convinced.

“This is meant to draw more people in, to create a hub, create more activity. And let’s face it, that’s what museums have to do now,” he said. “I understand the rationale for it, but I like the [Haupt] garden. It’s a little refuge, an oasis.”