Midtown East is home to the United Nations and to some of the ritziest real estate in Manhattan. But by some measures, it is also one of the borough’s most unattractive locations. The neighborhood district can claim the least amount of public open space in Manhattan, and is cut off from its waterfront by ramp spaghetti from the FDR Drive.

East Side elected officials and community leaders have been brainstorming for years over how to close a 24-block gap here in a potential East River Esplanade stretching from the Battery to Harlem. In 2007, the Municipal Art Society convened a charrette in which stakeholders and design professionals hammered out a bold vision for a new deck over the FDR Drive that connected via a slope to a new waterfront esplanade.

But now, what has been touted as a once-in-a-lifetime planning opportunity could be in danger of expiring. The immediate threat to any plan for closing the gap in the esplanade is the potential removal of a row of caissons in the East River.

The caissons served as supports for a temporary roadway that the New York State Department of Transportation built while they were working on the FDR Drive several years ago. Planners say the caissons potentially could be repurposed to serve as supports for a section of the waterfront esplanade that would stretch from about East 53rd Street to about East 62nd Street. Reusing the caissons could save $20 million to $25 million toward the cost of building this section. However, citing environmental concerns, the New York State Department of Environmental Conservation (DEC), which has oversight of the caissons, wants the city to either move forward on a plan for the East River Esplanade or remove them.

“The reality is that if some sort of agreement isn’t reached that would generate building part of the promenade over the existing caissons, it certainly would be a loss,” said Sarra Hale-Stern, district office director for New York State Senator Liz Krueger, who has been working with the city’s Economic Development Corporation to develop a proposal for the waterfront park. According to Hale-Stern, not only would the city lose the opportunity to save millions of dollars toward the cost of building the esplanade, there is also the possibility that DEC would not even allow installation of new supports.

The DEC has extended a March deadline for removing the caissons, but officials say these structures may require significant work to prevent them from eroding. “DEC has not yet set a ‘drop dead’ date, in order to allow for the possibility of the city using the caissons as part of its East River Esplanade design,” said Lori Severino, a DEC spokeswoman, adding that the city needed to show progress toward a design if the caissons were to remain.

The caissons are just one of the many hurdles to closing the gap in the esplanade. Financing the missing link could cost up to $200 million in a complex real estate deal that would radically reshape the Midtown East neighborhood.

But to make that plan work, two city-owned office buildings currently occupied by the UN would have to be sold. A popular playground that shares a full block site with a ventilating tower for the Queens Midtown Tunnel would also have to be demolished to make way for a new UN building.

“This conversation has been going on for ten years,” said New York City council-member Daniel Garodnick. “The question is whether you could come to a preliminary agreement that would allow the process to move forward at all.”