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.

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