Building in metropolitan Miami has long been limited by the lush Everglades to the west and south, and the Atlantic Ocean with its gentle breezes to the east, leaving precious little land available. Now developers want to break ground on a mostly deserted island in the warm blue waters of Biscayne Bay.

Watson Island, a mostly undeveloped, city-owned pile of silt that sits in the bay just east of downtown Miami, has proved elusive to builders for many decades. But Merrimac Ventures and BH3 Management are proposing to build a 10.8-acre mixed-use project on the 86 acres between Miami and Miami Beach, solely accessible by MacArthur Causeway.

“This is the most underutilized, rare opportunity that exists in all of Miami,” said Greg Freedman, co-CEO of BH3 Management, in an interview with CoStar News.

The proposal has attracted attention across Miami, where voters in November will be asked to weigh in on the proposal that could end a decades-long effort to develop the land. Even if it passes, the risk is high: a similar plan approved by voters in 2001 was never completed.

Watson Island is home to the Miami Children’s Museum, the zoological park Jungle Island and little else, leaving about 66 acres undeveloped. It's one of a handful of man-made islands scattered throughout Biscayne Bay that trace their history to the origins of Modern Miami's 1896 incorporation and the arrival of industrialist Henry Flagler's Florida East Coast Railway. While the area may have first gained international notice in the 1600s with tales of pirates drawn to the bay that has an almost tropical feel, it was the railroad that catalyzed a real estate boom that would last up until the 1920s.

Dozens of canals were dredged to drain and fill what was a swampy extension of the Everglades into a new kind of treasure — land suitable for building on, leading to the construction of thousands of houses, roads and even a railway that connected as far south as Key West. The island was deeded to the city in 1919, and reclamation was completed in 1926 as part of the construction of a bridge to Miami Beach and a replacement for what is today the Venetian Causeway.

Other islands were eventually developed, with Claughton Island becoming Brickell Key, home to luxury condos and the Mandarin Oriental; Star, Palm and Hibiscus islands becoming ultra-luxury single-family home neighborhoods; and Dodge, Lummus and Sam's islands morphing into PortMiami. But Watson Island, named after early Miami mayor, real estate developer and business owner J.W. Watson Sr., has remained mostly undeveloped beyond the relocation of the Miami Children’s Museum on about 1.2 acres and the 18-acre Jungle Island park in 2003.

“I think we're all tired of driving by the site and just seeing vacant land. We all love the Children's Museum. It's an incredible gift and gem in our community, but it sits there by itself, truly on an island,” Nitin Motwani, managing partner of Merrimac Ventures, told CoStar News.

The proposed mixed-use project from Merrimac Ventures and BH3 Management includes two hotels, 97,430 square feet of retail and 100,350 square feet of boutique office space, 105 condominium units, upgrading and expanding the seawall and boardwalk, infrastructure improvements, and a public park and promenade, creating what would be the city's newest neighborhood.

Assuming the vote passes, the developers will enter a 99-year ground lease with the city for the mixed-use project and contribute $9 million toward affordable housing in Miami. The agreement includes a commitment from the developers to hire locally for employees working on the project.

“The things that we're doing ... are refining what's already been approved, what's already been awarded at the site, and just making it more relevant for today's market,” Motwani added.

Watson Island is mainly made up of sediment and dirt dredged from the drainage canals that made it possible to build in Miami in the first place. As far back as the 1970s, city officials have been looking for ways to develop the island, and in 2001, the dusty island seemed poised for action when Flagstone Property Group won the vote to develop a nearly 11-acre site on the island.

Despite some initial delays, the relocation of the Miami Children’s Museum and Jungle Island in 2003 added to the impression the land was finally seeing some long-awaited development.

But Flagstone's plans never materialized because of a mix of factors, including the Great Recession and lawsuits filed by local residents. Only the seawall and marina were completed in 2016, and the following year, city officials voted to end the deal with Flagstone.

Miami, unlike cities in the American heartland, can't sprawl in any direction because it is hemmed in by the establishment of Everglades National Park in 1947, Biscayne National Park in 1980 and Miami's Urban Development Boundary in 1983. Building close to Florida's natural attractions often draws a fair share of scrutiny from environmental groups, even on projects within the development boundary. At the end of last year, for example, a proposed waterpark and hotel adjacent to Zoo Miami was withdrawn after conservationist groups and local residents raised the alarm about the project's potential harm to endangered bats.

That means as the region is increasingly built out, developers would rather go vertical and make the most of whatever land is already available than build in environmentally sensitive areas.

Miami has evolved in the 23 years since the original requests for proposals for Watson Island were released by the city, said Motwani. That has created an “opportunity to do something really special,” he said. The Watson Island project will be privately funded, and the developers want to make sure that “voters understand the facts,” he added.

Motwani, who helped lead development on the 27-acre Miami Worldcenter project, told CoStar News the team is taking lessons they've learned from Miami Worldcenter to implement in the Watson Island project.

“We're actually reducing overall development on the site, increasing public benefits, increasing contributions for affordability, increasing contributions for infrastructure and resiliency, increasing the overall aesthetics, public space and community-first hiring,” he said.

Moving forward, the team will be making sure that “our fellow Miami residents understand the opportunity that this is,” Motwani added. Freedman agreed, saying the team was working “around the clock” to deliver what was promised.

“It’s truly a win-win for all parties, between the economic development, the additional revenue to the city, the job creation, the public space.” said Freedman. “It’s a very rare opportunity to be able to create something of this scale and impact,” he added.