In the film “Upside Down,” two worlds exist opposite one another, one looking up and the other down, with the potential for a shared future.

Perhaps that’s too whimsical a description for an urban park and a garage below.

But the image loosely fits a new deal to construct Willoughby Square, the long-awaited public space that New York City officials consider Brooklyn’s answer to Bryant Park in Midtown Manhattan. An entirely invisible world underneath the park — in the form of a high-tech, mammoth underground garage — will in part finance creation of the aboveground square, an agreement negotiated by the city that resurrects a cornerstone of the extensive plans for rezoning the borough’s downtown.

The square will span more than an acre on Willoughby Street, a half-block from the Fulton Street mall, with manicured lawns, walkways and gardens, as well as a site to commemorate the abolitionist movement. Below it, the garage will house about 700 cars at any given time, relying on a system of computers rather than garage attendants. It will be the largest automated parking facility in North America.