At approximately 1:45 p.m. on Nov. 14, 2017, residents of 15 Alabama counties reportedly were shocked by a startlingly loud boom that caused some to call 911 operators in alarm.
As the National Weather Service's Birmingham station tweeted not quite a couple of hours later, there wasn't any clear explanation for the noise. Radar scans and satellite imagery of the region didn't show any large fires or smoke from an explosion, and the U.S. Geological Survey didn't spot any signs of an earthquake on its seismic monitoring system. "We don't have an answer, and we can only hypothesize with you," tweeted the NWS, which speculated that the sound may have been caused by an aircraft or a meteor.
But a NASA scientist soon knocked down those possible explanations. Bill Cooke, head of the space agency's Meteroid Environment Office at the Marshall Space Flight Center in Huntsville, told Al.com that he was skeptical that the sound could have been caused by a meteor, because there were no reports of anyone seeing a fireball. Cooke also discounted the possibility that the boom had been created by an aircraft, noting that the seismic signature "is not characteristic of that produced by a supersonic jet's boom."
The Elginfield Infrasound Array, 600 miles (966 kilometers) to the north in Ontario, Canada, picked up an infrasound wave that apparently was linked to the boom. It usually takes something pretty big, such as a severe storm, an avalanche or a rocket launch to trigger such a wave.
To add to the weirdness, less than two weeks later on Nov. 26, another, similarly unexplained boom was heard in the Birmingham area at 7:24 a.m. James Coker, director of the Jefferson County, Alabama, Emergency Management Agency, says in an email that he heard a double boom, "although the sound I heard may have included an echo caused by the mountains."
But Alabama isn't the only place where things have been booming lately. Skyquakes, as they're commonly called, recently have been heard across the U.S., in states ranging from New Jersey to Idaho, as well as in places as far away as India, where the seaside resort towns of Digha and Mandarmani were jolted in August by a boom so loud that it shattered hotel windows, according to the Dhaka Tribune.
Indeed, as USGS scientist emeritus David Hill detailed in a 2011 article on the subject, mysterious booms have been heard for many years in places across the world. In Belgium, they're known as "mistpouffers," while the Italians call them "brontidi." In the vicinity of Lake Seneca in the Catskill Mountains of New York, residents have long heard the "Seneca Guns," a phenomenon that was described by author James Fenimore Cooper back in 1851 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."
As Hill noted in his article, numerous explanations for skyquakes have been proposed over the years, including shallow earthquakes that could produce audible sounds without noticeable shaking, massive tsunami waves breaking far from shore, explosions of methane gas released from methane hydrate beds, sand dunes sheared by avalanches and, of course, meteors. As Hill wrote, meteors penetrating the upper atmosphere could create sonic waves that wouldn't reach Earth's surface until after the meteor had vanished, so that the connection between the two wouldn't be apparent.
People also have raised the possibility that the booms (or at least some of them) might be caused by tests of secret U.S. military aircraft, such as the long-rumored spy plane that aircraft buffs have dubbed "Aurora."
"I would imagine many of them are related to classified missions involving military aircraft," television meteorologist James Spann, whose Facebook and Twitter accounts became a clearinghouse for information about the Alabama booms, writes in an email.
In fact, there may not be one single explanation for all the booms.
"It is indeed the case that there may be several plausible explanations for any given incidence of a mysterious booming sound, and that the environment where the sound is heard will determine which of the possible explanations are most reasonable," Hill explains. "A sound heard in the middle of the desert, for example, is not likely due to breaking surf."
Hill says that most booming sounds are heard over a limited range, so reports of clusters of booming sounds separated by large distances are mostly likely coming from multiple sources. The exceptions "may include the sound from a meteorite exploding in the atmosphere high above the Earth, a massive volcanic eruption or an aircraft flying at supersonic speeds for an extended distance."