A manhole cover blew out of the ground around 8:30am this morning at Bush and Kearny streets, after an explosion in an underground PG&E electrical vault. According to the SFFD, no one was injured in the explosion (or by the manhole cover), but locals are being asked to avoid the area while PG&E addresses the issue, and there may be some traffic backups. 

UPDATE1 Smoke was visible initially no fire seen by SFFD. Expect traffic delay in the 300 block of BUSH 0846 hrs https://t.co/lpzxPlLvDl

— San Francisco Fire (@sffdpio) August 31, 2016

Curious what an electrical vault is, or why it might explode? This article from Popular Mechanics has more info. 

Update, 1:36pm: According to the Chronicle, the explosion was caused by a burst fire hydrant at Belden Place and Bush, which occurred around 6:30am. Water from the hydrant gushed into the underground vault and caused an electrical problem.