It's just as the name sounds, really: The Cross Florida Barge Canal is a canal project meant to connect the Atlantic Ocean and the Gulf of Mexico by going through Florida. Two sections of the project were completed (see the above picture) before the entire thing was called off for environmental reasons. The area around the barge canal is now a protected green belt corridor, about one mile wide in most places.
This project was in the works not for years or decades, but centuries. First proposed in 1567 by Philip II of Spain, the canal project was considered, for a long time, to be economically unfeasible. It was proposed again in the early 19th century as a means to cut back on the severe losses due to shipwrecks and piracy that afflicted boats navigating around the southern tip of Florida.
In the 20th century, President Franklin Roosevelt set aside funds to build the canal in 1935 as a public works program; He was putting people back to work during a tough economic time. Local opponents fought long and hard, bringing the project to a halt only a year later. The canal would deplete the aquifers across the state, the opponents argued. But in was only six years later, in 1942, that work on the project was being considered again. This time it was billed as a national defense project and a series of dams and locks were proposed to protect the underground water supply.
The project wasn't officially canceled until 1991 - in the years between, President Kennedy approved more funding, President Johnson started construction, and opponents continued to fight. Officially known as the Marjorie Carr Cross Florida Greenway, what remains of the project is named for the leader in the fight against canal construction.