At least one manhole cover dislodged at the intersection of Grant and California streets around 10:15 a.m. because of an underground electrical failure, according to PG&E spokeswoman Jana Morris.
The manhole cover was contained by the casting structure that holds it in place and did not hit any passing vehicles, she said.
When PG&E crews arrived on scene, some smoke was emanating from the manhole, but that smoke had dissipated as of 11 a.m., Morris said.
Investigators from the electrical company were on scene to determine the source of the explosion.
"At this time we don't know the cause," Morris said.
Fire crews also remained at the intersection, which was closed to through traffic.