South Carolina coast
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Residents of Charleston, South Carolina and the surrounding towns of North Charleston, Mount Pleasant, and West Ashley were all startled by an unexplained booming noise which shook windows and doors. The noise was reported on the morning of Friday, May 5th just after 8:30 a.m. While reports of the boom came in from all throughout the South Carolina Lowcountry, experts and law enforcement officials currently disagree about what the source of the sound might have been.

Minor earthquakes or other seismic activity were speculated to be the cause of the boom, but geophysicists reported little seismic activity in the area at the time. There are also armed forces bases nearby, and aircraft exercises were cited as the likely cause of a similar boom last year. However, military officials in the area claim that they weren't behind the noise.

That does not rule out any classified or undisclosed testing, however. The Air Force's secretive X-37B spacecraft landed in Florida over the weekend, causing a sonic boom upon reentry. Could there have been another secret landing we don't yet know about?

Maybe, but then again, these noises have a historical precedent outdating the U.S. Air Force. According to the United States Geological Survey, similar noises have been reported throughout the Charleston area dating back as far as 1886:
The sound appeared to come across the water of Ashley River from the west-southwest. Another observer of intelligence was seated in the park at the Battery, near the statue of Jasper. He suddenly became conscious of a deep murmur, which swelled in volume, and which appeared to come from the open bay, lying southeastward. Very soon there was a sound of agitation in the leaves of the trees overhead, and at the same instant, he thinks, he became aware of a tremor in the ground. Springing to his feet, there suddenly broke upon his ear a rapid swell in the sound, which became a mighty roar, and with the roar came a shock.
These mysterious booms have been heard for centuries throughout America's east coast, and are sometimes referred to as "Seneca Guns" based on a historical description by author James Fenimore Cooper. These booms were heard long before the advent of supersonic aircraft or naval artillery and are even sometimes heard near inland bodies of water. Their source currently remains a mystery. Could these recent noises in Charleston be yet another case of these unexplained noises, or is an undisclosed military test likely the cause?