Power was restored to most of the University of California, Berkeley, after a massive explosion left one person hospitalized for minor burns, and forced a campus-wide evacuation Monday night.

The school's Twitter feed announced early Tuesday that most classes would meet as scheduled.

Eleven buildings remained without power and classes in those buildings were canceled.

The explosion occurred in an underground utility vault outside California Hall before 7 p.m. Monday as crews were trying to restore power following an outage.

Witnesses described the blast as being a two-story fireball shooting into the air, KTVU reported.

In addition to the one person hospitalized, three others also received minor burns but refused treatment, said UC Berkeley spokesman Dan Mogulof.

"The explosion was about two stories high and as wide as a two-lane street," Mogulof said. "We're very fortunate there were no serious injuries."'

The blast may have been caused by an underground copper wire theft at a campus substation last week, Mogulof said. Officials thought they had repaired the damage Sunday.

"It appears that may have caused far greater damage than initially thought," Mogulof said. "It's more extensive."

Fire crews also freed about 20 people stuck in dormitory elevators as a result of the outage that also led to the mandatory evacuation on campus.

The power outage was reported around 4:30 p.m. Monday and led to evening classes being canceled and students told to suspend all lab activities, Mogulof said.

"I have midterms tomorrow as do a bunch of other students on campus. A lot of students don't have power to study or get their laptops on or anything like that," Kelsie Carlson, a freshman, told KTVU.

About a half-hour later, authorities reported an apparent ammonia leak in a room at Latimer Hall. It was contained almost immediately, campus officials said.

That was followed by the explosion, Berkeley Acting Fire Chief Gil Dong said.

The Associated Press contributed to this report

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