We were at the beach on the unseasonably warm Saturday and there was a boom loud enough it reverberated off the cliffs. It was as loud as medium-close thunder but there were no storm clouds. Off in the ocean almost to the horizon there was a large area of spray from the water that hung in the air for a couple minutes then settled. Everyone was pointing to the spot, and we talked with a few other people who had heard and seen the same thing and the predominate guess was "A boat exploded" but there was no smoke. A woman we were talking to called the police and they said they'd gotten a bunch of calls already. So the police show up, ask if it could have been a firecracker (we looked up videos online of firecrackers exploding under water and there just is nothing that would have produced an explosion that size. Nor could dynamite) but otherwise didn't have much to say. A few Coast Guard boats circled for a while then booked it out of the area, around the same time the helicopter and squad cars left. We thought for sure there would be something in the news but we haven't seen anything, and nothing on the police logs either. There were literally hundreds of people on the beach and everyone heard it and saw the area of spray. Like activity mostly stopped.

An additional point: there was an earthquake that we think was about the same time off Nantucket but it was very small and not that close.

Looking for theories and other people who witnessed the event or anything similar.