They say that no one was hurt.
Smoke was visible initially but no fire was seen, SFFD says.
People reported hearing a loud bang from the explosion in the Financial District.
SFFD says to avoid the area.
***AVOID AREA*** 350 BUSH PGE VAULT manhole cover blew off no injuries SFFD ON SCENE/ PGE ENROUTE/ NO FIRE 0828 pic.twitter.com/kbIZEMtjjr— San Francisco Fire (@sffdpio) August 31, 2016