Back in 1850, James Fenimore Cooper wrote a short story called "The Lake Gun," which recounted how people sometimes heard a loud, inexplicable explosive sound in the woods around Lake Seneca in New York. Cooper described it as "a sound resembling the explosion of a heavy piece of artillery that can be accounted for by none of the known laws of nature. The report is deep, hollow, distant, and imposing."
Since Cooper's time, people in various parts of the U.S. have been startled by similar booms -- though, when they get over their shock, they discovered that nothing appears to have been blown up, and no supersonic aircraft have been flying nearby. In 2012, for example, residents of Alabama, Georgia and Wisconsin all experienced shaking followed by loud booms. Scientists have speculated that the booms are probably caused by shallow earthquakes that are too small to be reported, yet large enough to be felt by people nearby. Or else, they may be sonic booms from planes traveling faster than the speed of sound. But nobody knows for sure [sources: USGS, Daniel].