Reports of smoke, steam, flames, and mysterious booms poured into the San Francisco Fire Department late Monday and early Tuesday, after a PG&E equipment failure caused a fire in a Tenderloin-area underground vault.

"It was like midnight and I heard a loud boom," area resident Kelse Schapp told ABC 7. "I thought it was a gunshot. I looked outside and everybody was on their balconies looking over. We just saw steam, it looked like steam coming out of the holes... weird."

Crews responded to the area of Hyde and Geary Streets, NBC Bay Area reports, after "witnesses saw fire and smoke shooting out of [a] manhole." According to PG&E spokesperson Abby Figueroa, by the time crews arrived, a small fire in one of PG&E's underground vaults had burned itself out, but left 351 customers without power until around 4:45 this morning.

Figueroa says the cause of the explosion and subsequent blaze was an "equipment failure," but couldn't comment further, saying that PG&E was still investigating the incident.

As of noon, Figueroa says, crews remained on the scene, repairing the "significant amount" of underground damage wrought by the fire.

According to ABC 7, area "traffic lights are back on because of a temporary fix, but a worker at the scene says there's a chance they will get turned off again" as they continue to work. So, if you can, avoid driving in the area, just in case additional repair work sends the signals back into darkness.