"There was some serious flames coming out of there," Fopiano said as he pointed to the intersection of Dunster Street and Massachusetts Avenue. "There was a lot of flames."
Fopiano was one of dozens who witnessed a persistent fire that sent flames up to 15 feet in the air, following a string of manhole explosions early May 2.
In all, four NStar manholes exploded beginning around 2:30 a.m., forcing partial closure of one of Cambridge's most famous pieces of real estate for about eight hours to both pedestrian and car traffic. MBTA Red Line service continued without interruption, and some bus service was rerouted.
Caroline Allen, an NStar spokeswoman, said crews were working to repair the burned and soaked wiring all weekend, hoping to avoid disruptions of the 24th annual Mayfair, which was held in Harvard Square May 4.
Allen said the cause of the blaze remains under investigation.
Deputy Fire Chief James F. Burns, a department spokesman, said the explosions occurred in three manholes running along the red brick sidewalk in the square and in a fourth at the corner of Dunster and Massachusetts Avenue.
He said that after the initial explosion, lubricating oil used in electrical transmissions caught fire, causing the heavy flames that poured out of the manhole.
Once certain that the problem was limited to the four manholes in the square, Burns said, firefighters allowed the blaze to burn itself out and then sprayed water to cool down the electrical conduit tunnel, so utility workers could begin repairs.
Harvard University classes were not disrupted, but Holyoke Center was closed. Harvard's infirmary was evacuated, and one student was transferred to a Cambridge hospital by ambulance, a Harvard spokesman said.
There were no injuries from the fire, and no substantial damage was reported to businesses in Harvard Square, officials said.
The cable malfunction was akin to a house blowing a fuse, but on a much larger scale, NStar's Allen said. The initial blast knocked out power to 700 customers, but service was fully restored by 12:30 p.m.