What was it? A few theories are floating about:

KIRO says, "Unconfirmed reports say the large explosion may have been from a plane crash in Napa Vine."

Meanwhile, joshtrujillo says, "Source here at Boeing Field says sound barrier might have just been broken by military planes high overhead."

And most conclusive! KING 5 News says, "FAA confirms sonic booms from military aircraft in the area."

As you were, everyone. Nothing to see here.