Already on edge days after two bombings in New York and New Jersey, residents were startled on Wednesday afternoon by an earthshaking boom that thundered across several neighborhoods in Queens and on Long Island.

Many jumped on Twitter to unleash a stream of panicked, anxious questions: Did you hear that? What happened? Had terrorists struck again?

Well, New York, there is good news and bad news. The noise that shook buildings and rattled nerves does not appear to have been caused by terrorism. But no one seems to know what, exactly, was the cause.