We're hearing reports of a manhole cover explosion happening in the city's Union Square area. KCBS tweets: "Black smoke reported at Post & Grant in SF, reports of a manhole cover explosion."

Joanna Gnome follows up with: "street explosion at Grant and Post st, cops and firemen galore. Not sure if it's gas or manhole that blew."

And MotleyLou described the scene thusly: "Loud BOOM then smoke. Manhole cover blown and landed feet away. Post and Grant."

As most of you know by now, Manhole cover explosions are nothing new to San Francisco.

We'll update as soon as we know more. Please use the comment section for manhole-explosion puns.