Crews worked overnight Monday into Tuesday to extinguish the fire that injured five people on Monday at a waste water treatment plant in Quincy.
Five workers were injured and were taken to Boston Medical Center for treatment of respiratory issues and eye irritation.
The fire continued to burn 100 feet below ground on Tuesday morning, hours after it first started. By 4 p.m. on Tuesday, more than 24 hours after it began, the fire was extinguished.
The treatment plant is part of the Massachusetts Water Resources Authority and is located on Nut Island, but the incident has no impact on the drinking water.
Crews initially had the fire under control. but it rekindled twice on Monday night.
Crews had trouble battling the flames because it was several hundred feet down.
MWRA chief Fred Laskey said the incident happened several floors down in the building when crews were working with an air scrubber.
"The goal is to let the firefighters fight the fire from the top down," Laskey said. "The issue is that the fire is inside duct work, so it’s very difficult; they can see it with the heat-sensing machine, but there’s still flames in there. And every time they find a way to get water into the duct, it simply pushes the fire farther into the duct."
A crane was brought in to aid the firefighter’s work.
Officials are investigating the cause of the explosion.
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