The 50- to 60-year-old steel cable probably “weakened until the point where it came down” after decades in a damp, subterranean tunnel, said Phillip Eng, the MBTA general manager.

Hundreds of passengers aboard the Blue Line were evacuated Tuesday afternoon after a train became stuck beneath the Boston Harbor, between the Maverick and Aquarium stations, officials said. Over the course of about an hour, Boston firefighters assisted with the evacuation of 500 people from the disabled train, the department said.

The MBTA's general manager said that a decades-old cable led to the shutdown of the Blue Line on Tuesday, which prompted the evacuation of 500 people under Boston Harbor.