Update December 16: Two days later, a 3.6 quake rattled the area.

It's not a huge quake, but big for our area.

First reporting: Around 12:30 a loud boom shook residents of Folly Beach and James Island, but as recently as 1:30 the Coast Guard, and local and county officials were still looking to find out the cause.

But, as there's been no public reports, it's looking to be that the incident may have been harmless.

Possible explanations being investigated are F-18s that were practicing in the area, and, perhaps, an undocumented reenactment.

One of our readers suggested that the boom could have been caused by the ocean's mystery noises. It's a mystery phenomenon, often referred to as "Seneca Guns."

Sonic booms have been known to roll off the ocean long before supersonic flight was discovered. The proposed explanations are far more numerous than any revealing evidence. Naval ships firing large guns, underwater landslides, methane gas escaping from the ocean floor, meteors, and sink holes are all proposed explanations for the phenomenon.

Writer palmettobug53 over at The Weather Underground writes:
You are sitting in your living room watching TV, reading a book or conversing with family or friends, when suddenly.... BOOM! Dishes in the china cabinet rattle. Perhaps a vase jiggles on the table. You look around, highly alarmed. That didn't sound like a simple sonic boom. Earthquake, maybe? But do earthquakes make loud booms like that? You don't know. You peer out the window or go outside and look around. Everything seems perfectly fine. You eventually return to whatever is was you were doing, still a bit perplexed.

From time to time, along the SC/NC coast loud booms are heard, with accompanying shaking of items inside homes. The phones at local TV and radio stations ring off the hook, with people calling to find out just what in the heck was that noise? Military bases frequently report that they had no one flying at the time. Seismograph stations report no tremors.

In fact, the noises were heard during Charleston's devastating 1886 earthquake. The USGS reports:
"For several weeks following the principal disturbance minor shocks continued to be felt at frequent intervals. Many of them would have been considered very forcible and alarming and they not been greatly disparaged by the convulsion of August 31. Almost all of them were accompanied by loud detonations. Mr. McGee thus describes several which he experienced.

I reached Summerville about 5 o'clock p.m. Detonations were heard at intervals averaging perhaps half an hour. From that time until 9:30 p.m. occasional and very slight spasmodic tremors of an instant's duration accompanied the detonation. I endeavored to determine the direction from which the sounds came, but no two individuals agreed. They seemed to me to come from the northwest. They were much like, but somewhat more muffled than peals of thunder at distance of half a mile or more, or perhaps more like the discharge of a blast in a mine or quarry at a little distance."

I am, of course, not suggesting the noise is connected to an earthquake. -- An innocuous -- but loud -- escape of methane gas seems far more probable.

The same tipster who suggested the ocean noise mentioned that, "About five years ago there was a series of seven or eight of these. It was kinda scary."

You can learn loads more about the phenomenon at the USGS.

We'll keep you posted.