The world is a noisy place. But some sounds shake you to your foundation. Mystery booms that come out of nowhere and have indiscernible sources scare people and leave them wondering, “What just happened?”

Every week, we find a few reports of “mystery booms” in the news. People report a surprising, loud noise that shakes their house and rattles their windows. This is not a new phenomenon. Reports of such sounds can be traced back through news accounts from over a century ago. But the immediacy of social media connections today allows us to readily share our observations. Therefore, occurrences of mystery booms appear as if they are more frequent. It is not clear if this is accurate or just the result of a generally more noisy cultural landscape. “End Times” commenters are ever-ready to use reports of mystery booms as an indication that something frightening is going on within the planet or that we have an extra-terrestrial threat of some kind. That’s a belief-based opinion and not a scientific one. We can make educated conclusions about the source of mystery booms through examination of characteristics of the events including time, weather conditions, frequency, and location.

Categorizing mystery booms

To begin, we can place mystery booms into different categories based on location:

  • Land-based booms
  • Coastal booms

and based on the frequency of the booms:

  • Single occurrences
  • Multiple occurrences over a defined time span
  • Regularly reoccurring

The full extent of all the potential causes of land-based booms may be undefinable but the most common include the following:

  • Explosions (accidental and deliberate, such as use of tannerite or fireworks)
  • Industrial noise
  • Train or truck-related sounds
  • Military artillery exercises
  • Sonic booms
  • Quarrying or rock blasting

I experience house-rumbling booms almost once a month from military training exercises some 7 miles away. The train transfer station in the valley is sometimes very loud in their activity of moving shipping containers on and off the cars. And empty tractor-trailers bumping over potholes or bridge joints can also be a nuisance noise. Communities may employ small detonations to scare away nuisance birds daily. Low-frequency noises like these travel much farther than high-frequency noise but weather conditions affect how far and in what direction sound travels.

Skyquakes

Booms from these causes might be traceable back to their source and some are recorded by equipment (quarry blasts) or have records of the event (explosions, military training). But most often, mystery booms remain a mystery because they are unexpected, fleeting, leave no trace, and are locationally hard to pinpoint. The term “skyquake” appeared in online usage around 2004 [though I’m uncertain about this date] to describe booms that have no readily discernible source. In addition to the potential causes provided above, skyquakes can be a result of atmospheric ducting of noise (from man-made explosions, or thunder) from far away. Under certain meteorological conditions, sounds may travel through the air in a preferred path or bounce off layers in the atmosphere, affecting distant receptors but not the people in between. Thus, the receptors (people affected by the noise) are at a loss as to where it originated.

Single booms are obviously harder to pinpoint. Explosions are also frequently one-off events whether they are manufactured or accidental. Cultural-related noises are more likely during weekends or holidays when people are blowing things up. It also helps to know the areal extent of the sound – if it was very local or heard across half a state. These days, many homeowners capture singular mystery booms on their home outdoor surveillance equipment. This recording can help determine the general sense of the sound and direction.

Incidents of homemade bombs or tannerite explosions are increasing. These are occurring in remote areas, such as the San Juan Islands of Washington (from 2019 to the present) and in more urban areas, like Chicago and Loveland, Ohio. Law enforcement attempt to investigate but only rarely are perpetrators caught.

Military jets are careful about creating sonic booms today but it still happens. Prior to its retirement in 2003, the Concorde passenger jet was a potential culprit. Supersonic jets flying offshore can sometimes be implicated if military authorities are willing to comply with the request to check records.

Bolides entering the earth’s atmosphere often explode. Meteor explosions might be visible as a flash in the sky but, if it’s cloudy or very sunny, the flash may be visible only on satellite imagery or not at all. A meteor explosion most often will not result in any debris reaching the ground. Small bits of space rocks enter our atmosphere daily, but they are big enough to make a loud sound over a wide area when they break apart. The explosion at Tunguska in 1908, which was an airburst of a bolide, was potentially the loudest sound on earth in human history (rivaled only by the Krakatau eruption of 1883) – both would qualify as the most massive skyquakes ever heard by people.

An interesting, but unconfirmed, idea about skyquakes is that coronal mass ejections from the sun can accelerate protons enough to cause shock waves in the earth’s atmosphere. Or, there may be sounds generated from interactions in the earth’s magnetosphere. These speculative ideas have gained popularity over the past 10 years as people report skyquakes on social media. Prior to this, the noise might have been brushed off or simply didn’t make the news.

Geological sources of noise

A common geological source of mystery booms are shallow earthquakes. These are more common than people think, even more than many geologists recognize. Multiple booms over several days, weeks, or years that can’t be pinpointed to any known source may be indicative of shallow earthquakes. I’ll expand on earthquake noise below.

Subsidence of abandoned underground mine voids can also create booming noises and possibly register as small earthquakes. In specific areas, booms can come from volcanic activity, landslides, or methane eruptions (underwater “mud volcanoes”, permafrost collapse, seafloor bubbles). Recently, karst activity underground has been implicated in mystery booming sounds, especially after a very large storm event that causes huge volumes of water at high velocity to travel through underground void space, possibly moving debris with it.

“Frost quakes” (cryoseisms) occur in temperate climates where water in rock fractures or joints freezes and expands causing rock slabs to split. These events due to ice expansion are accompanied by loud noise and sometimes evidence on the surface. Frost quakes are rare but can be considered under the right weather conditions. Spontaneous rock exfoliation can also occur under conditions of thermal stress, but these events are more likely to be extremely localized and easily discernible.

Brontides

The phenomenon of reoccurring booms is called “brontides”. They are also called “water guns” or “land guns” because they sound like cannon fire or rifle reports. The brontides phenomenon received scientific attention decades ago but the sources of the sounds are still unconfirmed. They are common enough around the world that they are often named for a place.

Some examples of these regularly occurring land booms include:

  • Moodus noises of Connecticut (generally agreed to be seismic-related)
  • “Brontidi” on the Italian peninsula
  • Hanley’s guns in Victoria, Australia
  • Booms reported by Bedouins in the Egyptian desert (possibly from sand dunes collapsing)
  • “Gouffre” reported in Haiti (a seismically active area).

Water guns are booms that occur along the edges of oceans, lakes, or river deltas. Explanations for water guns are just as speculative as for the land booms and include waves breaking offshore, landslides or sand movement on the river banks, or detonation of underwater gas. The particular feature of water guns is that the water surface may transmit sound.

Booms on the coast

The Barisal Guns are known from the Ganges delta area in India. Reports exist from the 1870s when explosives and cannons were nowhere around and firearms were scarce. The booming sound is sometimes solitary but can be in triplets. Barisal guns events were often associated with rain suggesting a possible explanation of far-off thunder transmitted long distances. Another idea based on the location was that distant volcanic or seismic sounds were transmitted over or reflecting off the water surface. Related to this, some scientists speculated that local shallow quakes resulted from the cumulative weight of sediment deposited in delta areas.

Similar booming noises have been reported all along the Eastern North America coast, from Canada to the Gulf of Mexico, often in the same general location over time. Here are some examples of other famous “water guns”:

  • Seneca Guns, Seneca Lake, New York. This phenomena is so well known that the media may refer to other instances of water guns as “Seneca guns”. Other lakes around the world have their own “lake guns” as well.
  • “Mistpouffers” are known from the coasts of northern Europe to Iceland, particularly Belgium and France. Their tendency to occur on warm, calm foggy days has earned them the translation of “fog dissipators”. A keeper of a lighthouse reported them with regularity. Interestingly, the sound was said to not be like artillery or thunder but this could be a result of atmospheric conditions that distort sound.
  • Uminari in Japan are described as the sound of waves breaking off shore. The lines of waves can produce a cannonading sound audible some ways inland.
  • “Lake Roar” from several Alpine lakes may possibly be related to karst caverns and wind.

Noisy earthquakes

Frequently, multiple booms reported over several months or years, or reoccurring periodically in a specific area can be linked to seismic events that are too small to feel, perhaps even too small to be measured.

Occasionally, you might find geological commenters who say that earthquakes don’t make noise. This is completely false. They make a lot of noise when you are nearby the epicenter. The energy released from the subsurface can manifest as sound when transmitted into the air. Shallow earthquakes may not create perceptible shaking at all and may only be noticeable from their booming sound.

John Ebel writes about earthquakes in New England:
“Observers near the epicenter of an earthquake magnitude 2.0 to about 3.5 typically describe the earthquake sound as a loud boom, like a nearby explosion or clap of thunder. If the earthquake is above about magnitude 5.0, observers near the epicenter often report a roaring sound that accompanies the earthquake shaking.”

The most famous place of “bad noises” in the US is Moodus, Connecticut, where seismic waves from tiny, shallow quakes are converted into sound waves in the atmosphere resulting in a very localized perception of booms. Moodus wasn’t the only “noisy” place noted in early America. Nashoba Hill in Massachusetts was known by the natives as the “hill that shakes.” Settlers eventually also heard the rumbling noises that came from the hill. The Native Americans were well aware of the recurrence of earthquakes throughout New England.

No place in the US is immune to at least small earthquakes. Earthquakes result from a situation where differential pressures in the rock exceed the rock strength. Mostly the stress is released through existing weak planes in the rock where movement occurred in the past (faults). Rock can break in an earthquake even if there is no fault present. Larger earthquakes take place in areas we consider to be seismically active, that is, along plate boundaries. So what explains earthquakes in the interior US that are not along plate boundaries? Today, the seismological evidence strongly supports the idea that the occurrences of earthquakes in northeastern North America are due to pressures in the crust that arise from the movement of the tectonic plates over the entire surface of the earth. Internal stresses still exist and prehistoric faults can still move, so these intercontinental quakes are hard to forecast. It’s not always clear why certain places experience unexpected swarms of small quakes accompanied by booming sounds.

Several towns that do not typically experience earthquakes were subjected to booms and nerve-jangling tremors over many months. Examples of such swarms included:

Residents of places where swarms occur naturally wonder if something bigger is coming (which does happen). Even if they are convinced that no strong shock is imminent, the frequent booms rattle nerves as well as dishes. Because the faults in the continental crust are short and the stresses are residual (from ancient events and not related directly to plate movement), the quakes in the interior US are not as large – not above magnitude 7. Due to the crustal composition, however, these eastern quakes transmit energy across huge areas and can produce widespread damage.

1791: Moodus, Connecticut
Residents around Moodus, Connecticut were no strangers to booming sounds. But the event on May 18, 1791 was far stronger than they had experienced before. It was felt throughout Connecticut, most of Massachusetts, and across Rhode Island. It is difficult to reconcile the magnitude based on reported damage effects and felt area but best guesses are around 4.5 to 5.0. Additional swarms of felt quakes occurred in August of 1981 and June 1982. These later, better recorded events, established that Moodus is an active seismic area. Its famous noises are the result of shallow, very local small quakes where the energy is converted to sound.

Solving mystery booms

What do you do if you experience a mystery boom? Firstly, note the time and characteristic of the event and the general direction from which you perceived it came. Check social media to see if anyone else heard it and has reliable information. Refrain from reporting to emergency services unless you notice a related surface effect (power outage, fire, etc.). Multiple events should be reported, but the officials will appreciate a detailed record of the occurrences. Local officials do their best to determine the source of local mystery booms. If the booms continue, residents expect additional action such as a higher level of investigation or calling in geologic experts. Trying to investigate mystery booms on your own is almost always futile and potentially dangerous.

It’s near impossible to list every potential cause for mystery booms and skyquakes; this piece was intended to give you a range of possibilities to ponder. Finally, to assuage any worries you may have, skyquakes and mystery booms may be spooky but they aren’t precursors to the apocalypse. They are often just the normal noises of the earth and of our civilization.

UPDATED: 4-Dec-2021 to note that Tunguska and Krakatau produced the loudest booms ever.

UPDATED: 19-Mar-2022 to include incidents of homemade explosives causing mystery booms.

Additional Info: The mystery booms and skyquakes of 2022

CHECK OUT MY VIDEO

Sources

Bressan, D. (2020). Seismologists Investigate Unexplained Skyquakes. Forbes, Dec 22, 2020.

Corliss, W. (1983). Earthquakes, Tides, Unidentified Sounds and Related Phenomena: A Catalog of Geophysical Anomalies.

Ebel, J E. (2019). New England Earthquakes: The Surprising History of Seismic Activity in the Northeast.

Gold, T. and S. Soter. (1979). Brontides: Natural Explosive Noises. Science, 204:4391. pp. 371-375.

USGS. Earthquake Booms, Seneca Guns, and Other Sounds.

Originally published November 2021 on SpookyGeology.com

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