We humans are pretty good at recognizing sounds. Our ears are sufficiently sensitive that they're capable of differentiating between variations in sound equivalent to less than one-billionth of atmospheric pressure, and our brains have the ability to identify and memorize complex aural patterns [sources: GSU, CNRS]. That ability probably evolved because our ancient ancestors had a better chance of survival if they could tell the difference between, say, the whistle of the wind and the hiss of a saber-tooth cat about to pounce.
But that innate skill at pigeonholing noises may be part of the reason why it's so unsettling to hear a sound that we can't identify. It doesn't help our anxiety level either that people have long associated mysterious sounds with paranormal phenomena, such as poltergeists.
Thanks to the advance of science, though, many sounds once categorized as "unexplained" are now identifiable. One example is "the Bloop," a mysterious noise that researchers recorded in the waters of the Pacific Ocean in 1997. The National Oceanic and Atmospheric Administration (NOAA) once categorized the origin of the Bloop as an "unknown." But in recent years, scientists have figured out that the Bloop probably was caused by the cracking and fracturing of icebergs, which causes tremors called icequakes [sources: NOAA PMEL, Newitz].
Other sounds remain a puzzle. Here are 10 for which scientists haven't yet come up with a conclusive explanation.
Back in 1977, just a few months before director Steven Spielberg's "Close Encounters of the Third Kind" was released, real-life scientists detected what they at least initially believed was a radio message sent by distant extraterrestrials [source: Kiger].
At Ohio State University's now-defunct Big Ear radio telescope observatory, which at the time was searching for such signals, a volunteer named Jerry Ehman noticed a signal that was extremely powerful -- 30 times louder than the typical ambient noise of deep space -- and extremely close to 1,420 megahertz, the frequency of hydrogen. (This was represented by the "U" in the printout of electromagnetic frequencies from the telescope. Ehman would scan these printouts every day.) But the signal only lasted 72 seconds, and more than 100 subsequent studies of that same region of sky failed to turn up anything unusual. Was the Wow! signal sent by a distant civilization with an extremely powerful transmitter, or just some natural anomaly? Decades later, we still don't know [sources: Wolford, Kiger].
Earth's oceans are a cacophony of often bizarre noise, caused by a variety of sources, ranging from volcanic tremors to ships, to aquatic mammals such as humpback whales [source: Bobbitt]. Scientist from the National Oceanic and Atmospheric Administration have been studying these sounds for years, with the help of an underwater listening network that uses hydrophones, which produce voltage signals over a range of frequencies as they pick up underwater sounds from any direction [source: Fox].
One mysterious unexplained sound is the Upsweep, a set of narrow-band upsweeping sounds, each lasting several seconds and first detected by scientists in 1991. The Upsweep seems to be seasonal, and usually reaches its peak in the spring and fall, though no one is sure why. The source level is high enough to be recorded throughout the Pacific [source: PMEL].
In the summer of 2013, a woman in British Columbia began hearing a loud, trumpet-like sound, and one morning she recorded it with her video camera and posted it to YouTube. While some questioned its authenticity, there have been other reports of a similar sound from Texas to Norway.
There are some variations: Sometimes the noise sounds more like the moan of an animal, while other manifestations resemble a low-pitched rumble, a whine or a thumping. Nobody has determined what it is, though University of Saskatchewan physics professor Jean Pierre St. Maurice has hypothesized that the noises may be from electromagnetic waves coming from aurora, natural light displays mostly seen in the Arctic and Antarctic skies [source: Huffington Post].
During the Cold War from the 1950s to the 1980s, shortwave radio enthusiasts across the world began noticing weird broadcasts that would often start with music or the sound of beeps, which would be followed by even more strangeness -- the voice of a woman counting in German, for example, or a child's voice reciting letters from the alphabet in English. Listeners gave them amusing names, such as "Nancy Adam Susan," "The Swedish Rhapsody" or "The Gong Station." Listeners assumed they were signals for secret messages to spies [source: Sorrel-Dejerine].
One of the strangest was "The Lincolnshire Poacher," which used a snippet of an English folk song of that name. After about 10 minutes of music, a female voice with an English accent would read what appeared to be a coded message. According to Dutch cryptology historian Dirk Rijmenants, the Lincolnshire Poacher appeared during the 1970s and aired daily until 2008, when it mysteriously disappeared. The common assumption is that it was some sort of communication between British intelligence and agents in the field, but there's never been any official confirmation, so it still qualifies as unidentified.
If you thought the Lincolnshire Poacher was weird, the story of UVB-76 is even weirder. According to Wired U.K. reporter Peter Savodnik, starting in the early 1980s, a mysterious radio tower north of Moscow transmitted a bizarre assortment of beeps, and then in 1992 switched to buzzing sounds that each lasted about a second and occurred between 21 and 34 times per minute. Once every few weeks, that routine would be interrupted briefly by a male voice reciting brief strings of numbers and words, usually Russian names such as Anna and Nikolai. The tones, amplitude and pitch of the buzzing shifted, and the intervals between it would vary as well. But every hour, on the hour, the station quickly would buzz twice.
Even more oddly, after years of daily broadcasts, the station briefly stopped sending out signals in June 2010 and again in August of that year. Then, toward the end of that month, UVB-76 suddenly underwent a startling metamorphosis, with thuds and shuffling sounds creeping into the broadcasts, frequent interruptions by snippets of "Dance of the Little Swans" from Tchaikovsky's "Swan Lake," and occasional recitations of cryptic messages such as "04 979 D-R-E-N-D-O-U-T" followed by long strings of numbers. Again, the assumption by shortwave buffs is that the station is transmitting messages to secret agents [source: Savodnik].
For decades, paranormal believers have been picking up stray voices on tape recordings, which some think come from dead people or extraterrestrials. As a website for paranormal enthusiasts explains, picking up electronic voice phenomena, or EVP, often requires a person to listen to the recording with headphones and work hard to pick the voices out from the background noise, and sometimes a recording will have to be played in reverse for the message to appear [source: ATC]. (A similar technique was used by teenage Beatles conspiracy theorists in the late '60s, when they played "Strawberry Fields Forever" backward in an effort to hear what sounded to some like "I buried Paul.")
But not everybody believes EVP is truly paranormal. The Skeptic's Dictionary, for example, offers effects such as interference from a local CB operator, and various sorts of naturally occurring electronic distortion as possible explanations.
This one is unexplained, in that nobody knows who did it or how they did it. On Nov. 22, 1989, a hacker managed to take over the frequencies of two Chicago-area TV stations. The first time, during a CBS newscast, technicians shut it down in 30 seconds. But the second attack, against a public TV station, lasted well over a minute. The PBS drama cut out and a man in a Max Headroom mask appeared, broadcasting a cryptic audio message, with lines like "your love is fading" and a hummed snippet of the theme song from the cartoon "Clutch Cargo" [source: Finley].
An Federal Communications Commission investigator said that whoever did it could have used simple gear, but had to have a sophisticated knowledge of broadcasting technology.
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].
Back in 2009, the Cassini spacecraft picked up Saturn Kilometric Radiation, or SKRs, natural radio signals being emitted by Saturn. What was bizarre was the pattern of the sounds, which they couldn't explain -- it's actually two signals, one from the planet's north pole and the other from the south pole, in an out-of-sync duet [source: Space.com].
While these sounds normally would be inaudible to human ears, scientists have altered them to create really spooky-sounding recordings. (They sound a bit like bursts from an air raid siren.) As University of Iowa scientist Don Gurnett, who headed Cassini's instrument team, told Space.com, "These data just go to show how weird Saturn is."
Taos, New Mexico isn't the only place where a bizarre ambient hum is heard, but it's probably the most famous spot for the mysterious droning, so much that it's mentioned in travel guidebooks about the area. The Taos Hum reportedly is heard by only 2 percent of the local population and been described as a sound similar to the low rumble of a diesel truck's engine [source: Eisenberg].
People who noticed it are bothered to varying degrees, with some experiencing a mild feeling of irritation, while others report sleep disturbances, dizziness and nosebleeds. Conspiracy theorists suggest that it's caused by some sort of secret military communications system used to contact submarines, or a clandestine weapons testing program.
However, scientists think it may be caused by low frequency waves that originate in the atmosphere or else by vibrations from deep within Earth. A third hypothesis is that some people are extraordinarily sensitive to certain electromagnetic frequencies, and that the hum is caused by devices such as cell phones. But again, nobody really knows for sure [sources: Strange Sounds, Eisenberg].
What is a 'smocking gun'? HowStuffWorks investigates.
Author's Note: 10 Unidentified Sounds That Scientists Are Seriously Looking Into
I can't say that I've experienced any of the unexplained sounds described in this article—I was in Taos once during the mid-1980s, for example, and the only odd phenomenon that I noticed was a local artist who wore cowboy boots that he'd apparently spray-painted fluorescent orange. When I started working in downtown Baltimore a few years later, I remember that on one of my first afternoons on the job, I was startled by the loud, eerie wail of a siren. No one else in the office seemed to notice it. I eventually discovered that it was the weekly test of an emergency alert system, and sounded at 1 p.m. each Monday. In time, I became accustomed to it as well.
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