The town of Taos, in north-central New Mexico, has been home to many famous residents including Julia Roberts, Dennis Hopper, D.H. Lawrence, Aldous Huxley and Donald Rumsfeld. It's a small, laid-back artsy community that is also home to an unusual mystery: a resident hum of unknown origin, the so-called "Taos Hum."

A variety of theories have been offered as an explanation, ranging from the mundane to the fantastic, the psychological to the paranormal. Stoned hippies, secret government mind control experiments, underground UFO bases and everything in between have been blamed.

The hum seems to have first been reported in the early 1990s. Joe Mullins, a professor emeritus of engineering at the University of New Mexico, conducted research into the Taos Hum. Based on a survey of residents, about 2 percent of the general population was believed to be "hearers," those who claimed to detect the hum. Sensitive equipment was set up in the homes of several of the "hearers," measuring sounds and vibrations but after extensive testing nothing unusual was detected.

The research revealed, however, that there was not a single identifiable Taos Hum but instead several different ones that people reported; some describe it as whir, hum or buzz. The fact that not everyone heard the same thing was puzzling, and suggests that they may have been reporting subjective experiences instead of objective sounds.

Mystery sounds

Mysterious sounds are nothing new, of course, and Taos is not the only place plagued by unknown noises. In fact, there are dozens of other cities, both big and small, in which some residents claim to hear something strange.Just as there may be more than one explanation for unidentified lights in the sky or along the skyline (including aircraft, clouds reflecting spotlights, car headlights and so on), there may be more than one explanation for unidentified sounds. The mysterious sounds range from a high-pitched squeal to a low murmur to a faint rumble, and the explanations are almost as varied as the sounds themselves.

Though some of these mysterious sounds remain unexplained, many were eventually identified. For example, in 2012 residents in Borneo reported hearing bizarre roaring or snoring sounds beginning very early in the morning and lasting a few hours until dawn. It happened two days in a row, frightening and puzzling locals. Investigation revealed that the mysterious noises were caused by a nearby factory testing their boiler while the plant was closed. In February 2014, a mysterious short-lived "loud droning," "unearthly" sound like in the science fiction film "Independence Day" was reported in the skies over Coventry, England. It baffled residents for miles but an unseen airplane was later revealed to have been the cause.

Some reports of unexplained sounds were later revealed to be hoaxes. For example, a video posted to YouTube in January 2012 by a young Canadian university student near Edmonton, Alberta, contained strange sounds that she asked for help in explaining. The mysterious sounds video went viral, and garnered nearly 2 million views before it was exposed as a prank. In an interview with a local newspaper, she admitted that she made the video "to show my friends and family how easy it was.... and how they shouldn't believe everything they see online."

Physical and psychological explanations

Humans live in a constant sea of background noise, most of it unnoticed until we start paying attention to all the sounds and focusing on them. While many people may assume that locating the source of a sound should be easy, in practice it can be very difficult. There are hundreds of potential sources of noises including traffic, boats, planes, insects, large machines, wind, freight trains, mining and other industries. It's not as simple as listening for a sound and walking toward it until you find its source.

Many researchers suggest that the answer to the Taos Hum mystery may be found in the inner world of personal experience instead of the outer world of factories and heavy equipment. What does your tongue taste like? What does your nose smell like? What does your ear sound like? These are not silly, simple questions but instead may hold part of the answer.

Even though we don't notice it, our ears sometimes create their own noises. And because the sounds are subtle (and because most people are constantly surrounded by sound, whether it's music, television, video games, or just a typical noisy city life) we don't hear them until it's very quiet or we are listening carefully. This phenomenon, called spontaneous otoacoustic emissions, is different than auditory disorders such as tinnitus, which causes a ringing in the ears. It might explain some of the "hearers" reports.

The fact that only a tiny percentage of people claimed to hear the Taos Hum was also puzzling; it's not that the other 98 percent of the Taos population had poor hearing, but instead perhaps that those who heard the hum were "super hearers" with unusually keen hearing. Or, it is also possible that, given such a weak effect in such a few number of people (and whose descriptions of the hum do not always match up) that the hum is merely an auditory hallucination. Such hallucinations do not necessarily indicate any sort of mental illness or disturbance, but may simply be the result of common (and harmless) psychological and physiological processes. Neurologist Oliver Sacks, for example, has written extensively on both visual and auditory hallucinations in his books "Musicophilia: Tales of Music and the Brain" (2007) and "Hallucinations" (2012). Some of the Taos Hum hearers have even reported hearing it after they have moved out of the area.

As always in science, "unexplained" does not mean "unexplainable." Countless things about the world around us were initially mysterious and unexplained (from the causes of germ-borne disease to the nature of lightning), but were eventually explained through research and science. It's possible that the Taos Hum is real, and its true origins remain unknown, and it's also possible that the hum only exists in the minds and ears of those who report it.

Benjamin Radford is deputy editor of Skeptical Inquirer science magazine (www.csicop.org) and author of six books, including "Scientific Paranormal Investigation: How to Solve Unexplained Mysteries." His website is www.BenjaminRadford.com.