The latest case happened in Pacific Heights Saturday night. At least one massive explosion shattered more than a dozen windows in that neighborhood.
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"It literally sounded like a car bomb or something, and it literally knocked me to the floor," said David Chang, a San Francisco resident.
"Probably the loudest explosion I've ever heard other than outside fireworks," said Jon Shubin who was visiting San Francisco at the time of the blast.
VIDEO: Underground explosion damages car, several windows at Pacific Heights in SF, fire department says
Underground explosion damages car, homes in SF: fire department
PG&E described what happened Saturday night as an underground equipment failure that firefighters say led to an explosion and fire. But while PG&E was working below the street Sunday, the company gave no update as to what led to the mishap.
Video shows Guneet Kaur's Tesla sitting right on top of fire that shot up through a manhole.
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"When I got there I saw it just explode and it got on fire," said Kaur.
"I did hear that this started after a manhole exploded under a Tesla, then I heard two other manholes exploded and windows were blown out of the building right here," said Catherine Stefani who is San Francisco District 2 Supervisor.
Firefighters say no one was hurt but this marks at least the second time in less than three months that there there has been what is called an "underground vault explosion" in San Francisco. An April case knocked out power for some businesses in North Beach and the Financial District for several days.
In both situations the power of the explosions were incredible.
VIDEO: SF business says they will lose $50,000+ due to power outage caused by underground fire
SF business says they will lose $50,000+ due to power outage
"A really loud bang. A series of them but the first one was really loud, really loud!" said William Laan of San Francisco.
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"It just blew out all the glass and all the windows," said Chang.
"When I realized there were broken windows, and a car was on fire, and fire was coming from the ground it seemed like something more serious," said Ben Short who witnessed the damage.
While just 20% of utility lines are underground nationwide, that number is growing, and is greater than 50% in densely populated San Francisco.
Mike Beehler of the Power Delivery Intelligence Initiative is an advocate for underground power distribution and reacted to news of the two explosions.
"The challenge J.R., is that these systems are old, they've been in there 30, 40, 50 years and that equipment gets old. Sometimes they do reach end of life and sometimes they do fail catastrophically," said Beehler.
MORE: SF apartment residents recap ordeal with days-long power outage following underground fire
In a statement sent to us Monday night PG&E says, "The root cause is under investigation" referring to Saturday's Pac heights explosion and fire. Witnesses described multiple explosions that damaged the Tesla and broke at least a dozen windows in the area.
"Utilities do a great job in maintaining reliability. They try to do preventative maintenance but sometimes things just happen and they'll happen and it causes an explosion like it did in San Francisco," said Beehler.
Beehler says that underground power distribution is safer than above ground wires despite the two San Francisco incidents in less than three months. Professor Daniel Kirschen of University of Washington agrees.
"It makes for a more reliable electricity supply because you have less things that can go wrong, but when they do go wrong you are in a situation like what happened," said Professor Kirschen.
In the April case some were without electricity for a week and last Saturday the damage to the neighborhood was extensive.
Here is the full statement issued Monday from PG&E:
"The safety and security of our employees, assets and facilities are our highest responsibilities. On Saturday evening, PG&E received reports of a loud noise and smoke from a manhole in the Pacific Heights neighborhood of San Francisco. PG&E crews responded and confirmed an underground transformer failure and smoke from an underground vault at 2143 Pacific Ave., San Francisco. 154 customers lost power around 9:15 p.m. Saturday and were safely restored at 2:30 a.m. Sunday. The root cause is under investigation."