The one between 19th and 20th Streets. "As far as I could tell, firefighters seemed to be putting out a routine fire," event planner Austin Riggs, who took this picture of the explosion as he was walking back to his office after a trip to City Bakery, told Daily Intel. "And then all of sudden there was this huge flame ball. I could feel the heat on my face."
Update: Firefighters have told Fox NY that it was a "manhole explosion" that "sparked a transformer fire" (we assume they mean an electrical transformer, not the toys). Bally's Total Fitness and Apex Technical School were also damaged.
Riggs also took video of the aftermath of the explosion. Note how unperturbed the cars and bike messengers driving by seem to be.