Ask SFist: Loud Boom in the Upper Haight?

Reader Michael wrote to SFist this morning to ask:

There was a loud BOOM in the upper Haight last night around 9:30, and this morning Haight was closed between Masonic and Ashbury. There was a large plume of what looked like steam rising from the street, which had been fenced off w/ flood lights shining into the cordoned-off area. Any news?

What on Earth happened, Haight Street denizens?