Swimming in Boston Harbor in wintertime. Hiking tree-lined trails under a translucent dome. Renting electric cars for a spin around Thompson Island.

It definitely seems like something out of a sci-fi book, perhaps a vision of life in Boston in 2020. But rather than some futuristic view of the Boston Harbor Islands, this was a proposed plan for developing the islands in the 1970s. Believe it or not.

Plans were drawn up in the early 1970s to stage a huge international exposition in Boston in 1976 to celebrate the American bicentennial. The planners of Expo ’76 projected that up to 60 million people would visit the 690-acre fairground, which, in addition to Thompson Island, would have been located on Columbia Point in Dorchester and a new harbor island created from a combination of landfill and floating platforms.

The plans called for construction of a 500-boat marina and a hotel on the southern end of Thompson Island as well as the conversion of the school facilities into a youth camp and demonstration farm. Visitors would have been able to rent bikes or electric cars to traverse the island.

The piece de resistance, however, was the proposal to build a huge, transparent dome, more than two football fields in diameter, that would have covered part of the islands’ natural landscape of grassy knolls, woods, and winding trails. The dome would have been climate-controlled, allowing for boating, swimming, and picnicking year-round, even in the throes of a New England winter.

Luckily, these plans never came to fruition, and the landscape of Thompson Island has been preserved. As is often the case, it’s everyday city residents who are partly to thank for the plan’s demise.

This Thursday (July 28), the Friends of the Boston Harbor Islands are putting on a program at the Thayer Public Library in Braintree exploring the World’s Fair that wasn’t. The program will include a screening of a public service film made by Save Our Shores, a grassroots organization, as well as a panel discussion with some of the film’s creators. There will be exhibits on display as well. Click here for more on the program.

Photograph by Sue Scheible, the Patriot Ledger

The plans for the 1976 World’s Fair were some of the most compelling items I came across in researching Discovering the Boston Harbor Islands. Always interesting to see an alternate history for how Boston could look today. Electric cars and swimming under a giant dome sounds pretty cool, but I’ll take the walking trails and open skies of Thompson Island just how they are.