Michael Maykrantz was on duty at a fire house on 74th street in Ocean City when the floor began to shake and the doors started to rattle.
At Bart Rader's house in Ocean Pines, a loud boom "like somebody blew something up" preceded shaking so heavy that it rattled a 50-pound metal sculpture against the wall.
Miles away in Annapolis, Ocean City Mayor Rick Meehan was meeting in state Sen. James Mathias' office when he got a text message from his daughter: "What the heck was that?"
A series of tremors rattled residents across Ocean City and the lower Delmarva Peninsula around midday Thursday, puzzling geologists and emergency managers. Within a few hours, geologists ruled out an earthquake, and by Thursday evening, signs pointed to supersonic jets flying from the Patuxent Naval Air Station.
Air station officials said Friday two jets were in the air off the coast at the time the rumbling was felt, and that weather conditions made it likely that sonic booms could have traveled further than normal.
The phenomenon nevertheless mystified many, including Maykrantz.
"We've had sonic booms in town before, but this seemed different," said the firefighter and paramedic. "It was more sustained, and then there was a pause for about a minute and then it started again."
Ocean City police dispatchers received more than a dozen calls by noon from residents reporting a loud boom and violent shaking shortly before noon, though a police spokeswoman said she didn't feel anything.
Most immediately thought of an earthquake, remembering the 5.8 magnitude temblor centered in Virginia on August 23, 2011, that also shook the Delmarva peninsula.
"We are hearing reports of a possible earthquake in Ocean City, MD. That is unconfirmed at this time," the Maryland Emergency Management Agency posted on Twitter.
The Maryland Geological Survey quickly confirmed sensing the rumbling on its sensors in Reisterstown. To triangulate the source or magnitude of a possible earthquake, they conferred with seismologists in Virginia, Delaware, Pennsylvania and New Jersey, said Maryland Geological Survey Director Richard Ortt.
"When the Earth moves, you get certain kinds of waves," Ortt said.
The only earthquake on record in Ocean City occurred Oct. 15, 1928, according to the Maryland Geological Survey.
But the seismographs didn't show the "signature" of an earthquake, Ortt said — a buildup lasting a few seconds before the violent shaking began, and then secondary and tertiary waves afterward. In this case, that pattern was not evident.
Scientists at both Columbia University and the University of Maryland, College Park, confirmed that no earthquake took place.
Yes, a frost quake. The phenomena known as cryoseisms have produced shaking and the sounds of exploding bombs across the northern United States this winter. They can occur after frigid weather, when frozen soil, rocks or water violently crack, and have been known to reach magnitudes as high as 1.5.
But that's not what happened in Ocean City where temperatures were close to freezing much of the day. Frost quakes have been common this winter in the iceboxes of Minnesota, Illinois and Canada.
That left some sort of man-made blast or boom.
Quarry blasts or surface explosions contrast with earthquakes on seismograph readings, appearing as sudden temblors, Ortt said. But there were none known of nearby.
Sonic waves also are commonly known to be mistaken for earthquakes, but officials at Dover Air Force base to the north and NASA Wallops Flight Facility to the south on the Delmarva said they had no activity to report.
Late in the day, the Navy owned up to scheduling two supersonic flights off the Atlantic coast Thursday from its test flight facility at Patuxent Naval Air Station in Southern Maryland.
That explanation satisfied the Maryland Geological Survey.
"At this time, we are confident that the supersonic flights were the cause of the felt reports," Ortt said in an e-mail about 6 p.m. Thursday.
Base spokeswoman Connie Hempel said Friday that the jets, an F-35C and an F/A-18, were in the air from about 9:20 a.m. to 12:20 p.m. along a strip of airspace that runs along the Delmarva, up to 12 miles off the coast and 3 miles away at its closest. Weather officials at the base said the sonic booms the aircraft created could have likely traveled further than normal because a temperature inversion was in place, when there is warm air aloft holding colder air in place below, with calm winds and recent passage of a cold front, Hempel said.