What surrounds the town of Springfield in "The Simpsons Movie"?

A giant dome surrounds the town of Springfield in The Simpsons Movie. The dome is placed over the town by the Environmental Protection Agency after Springfield’s polluted lake becomes an environmental crisis.

The problem begins when Homer Simpson dumps a silo full of pig waste into Lake Springfield. The pollution becomes so severe that the EPA, led by Russ Cargill, decides to isolate the town from the outside world. In the film, a gigantic glass dome is transported into position and sealed over Springfield.

The dome creates the movie’s central conflict: Springfield’s residents are trapped, communications are disrupted, and panic spreads as supplies run short. The Simpsons escape through a sinkhole beneath their house, eventually working to expose the EPA’s plan and save the town.

The answer is sometimes shortened to “a glass dome,” which is more precise, but “a giant dome” is the canonical quiz answer. The dome is a film-specific plot device and should not be confused with the recurring environmental jokes about Springfield’s lake in the television series.