An underwater property in Florida costs $43 million. The developer calls it a "unicorn".

Four acres of beautiful waterfront land in Boca Raton, , is on the market for $43 million.

There's only one problem: it's mostly underwater.

Image credit: William Swain via The Real Deal

Bringing new meaning to the term "underwater mortgage", Director William Swaim of Waterfront ICW Properties is proposing the submerged land for the development of single family homes.

Swaim calls it the "unicorn" because of its location and relatively low price. The plot sits along the Intracoastal Waterway, a series of creeks and canals that wealthy residents use to moor their boats and yachts. House prices in the area have appreciated monthly by more than 10% over the past 18 months. A mansion can cost up to $18 million per capita.

Swaim, who specializes in buying and reselling flooded land, said the area is "Boca Raton Intracoastal's last and largest undeveloped property for sale in 30 years."

But he also warns that anyone buying the property will have their work cut out for them.

"Submerged packages are a headache," Swaim told The Palm Beach Post. "It takes years to clean them, and most people don't want to take years to clean them."

His company will build a dike and fill in the land for an additional $3.5 million.

Swaim's land wasn't always under the sea. The property was above ground until 1957, but a seemingly greedy neighbor stole the land to build a housing estate nearby, Swaim told The Post.

Some local activists are concerned that over-development of properties like these could harm wildlife.

“Manatees are dying at a record rate due to impacts on their habitat, and increased development on submerged land will only compound this problem,” said Lisa Interlandi, executive director of Everglades Law. Center.

But a 2018 Army Corps review concluded that land development would not adversely affect sea turtles, smalltooth sawfish or endangered manatees and threatened.

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