It must have been the construction site across the street. Or maybe it was a tractor-trailer lumbering by, a train rumbling underground or even a pesky friend shaking one's beach chair.
Everything, that is, but what it actually was: an earthquake, so rare in these parts that when one emptied offices and homes, shook merchandise off store shelves and roused lazing sunbathers on Tuesday afternoon, it wasn't the first explanation that popped into many people's heads.
"Who would have thought there would be an earthquake in Baltimore?" asked Zinda Morris, 22, a member of a city Circuit Court jury pool.
With little serious damage reported, the earthquake in the Baltimore area was an initially unnerving experience that turned out to be more of a summer weekday distraction, a ready-made talking point for strangers thrown together on a sidewalk.
In today's climate, thoughts of a terrorist attack crossed more than a few minds as buildings started rocking, but just as quickly such fears proved unwarranted. Story after story began with an eye-widening, "I thought it was …" only to end in a shrug-inducing denouement.
"My house shook from side to side, and pots and pans rattled on the racks in the kitchen. I thought my flatscreen was going to rock off the counter," said James Gordon of Baltimore, vacationing in his townhouse in Rehoboth Beach. "But it did not."
It didn't take long — less than an hour, actually — for the Power Plant Live entertainment complex in the Inner Harbor to start promoting "Tremor Tuesday" drink specials.
The earth's shaking didn't rattle as much as confuse. Linda Jackson, driving from her West Baltimore home to her job as an environmental services worker at Mercy Medical Center, thought her minivan was having some sort of breakdown.
"It shook and shook," Jackson said. "I didn't have a clue what was happening. I thought my transmission was going up."
Once she got to work, she saw her co-workers on the sidewalk outside the downtown hospital, prompting misconception No. 2.
"I thought there had been a bomb threat," she said.
In Harbor East, the namesake products at Handbags in the City began falling off shelves and lights started flickering as Bree McNerney sat behind the register, mystified. As the trembling intensified, though, she bolted.
"I was out the front door," the 22-year-old said.
Along Frederick Road, the main drag in Catonsville, people spilled out onto the sidewalk to figure out what had happened. Pete Kriscumas, legislative assistant to Baltimore County Councilman Tom Quirk, was in a car parked behind the district office when the shaking started.
"It was really rocking," he said, as if someone was bouncing the back of the car.
"We looked in the [rearview] mirror," Kriscumas said, "but there was nobody there."
Similarly in Rehoboth, Michael Cormier of New York City suspected mischief as his chair "rocked back and forth" as well.
"I thought someone had come up from behind and was shaking it," he said.
But who? Maybe theelephants could have told him.
At the Maryland Zoo, general curator Mike McClure said the four Africanelephants must have picked up infrasound waves, which are lower in frequency than a human can hear and are believed to serve as early warning signals that earthquakes, tsunamis or other natural disasters are approaching.
The three females and the 3-year-old calf Samson "put their butts together, wrapped their trunks together and made a big ball of elephant," he said.
But the polarbears, coolly enough, slept soundly in the 2,000 pounds of snow brought in for a "Beat the Heat" event, McClure said.
That was what drew David Silver ofTowson and his family to the zoo Tuesday, and they were delighted to get an extra attraction as well.
"We got to see the polarbears, have free ice cream, and then we got a free earthquake," Silver said. "That's some entertainment they've got there."
The bears weren't the only beings oblivious to the quake.
Three window washers, suspended by ropes high above the ground, squeegee-d away at Mercy Medical Center's panes of glass even as crowds evacuated buildings around them. Drivers on the Orleans Street bridge over Calvert Street even rolled down their windows to alert them to the earthquake, to no avail.
"I thought they were messing with us," said Eric Ruhl, when he and his co-workers finally propelled themselves down to the ground at about 3 p.m., having finished a bank of windows.
None of the three workers for PSI of Harrisburg, Pa., hired to wash the windows, had any idea there really was a quake down below — perhaps, as Ruhl speculated, because their ropes stretch and have a lot of "give and bounce."
"Now that is scary," Adam Thomas said in hindsight.
"It was kind of funny, I saw everyone standing on the street," Michael DeJesus said, remembering that he thought it was just a late lunch crowd.
Even on the ground, there were those who missed all the excitement.