A mysterious illness has been striking people associated with the US Embassy in Cuba — and a secret sonic weapon is rumored to be the source. Over the past year, diplomats in Cuba have experienced an unusual collection of symptoms that range from hearing loss, vertigo, and nausea to concussions, CBS News reported.

Yesterday, the mystery grew even more complex when the Associated Press reported that the number of US victims has climbed to 21 people. Canadian diplomatic households were affected as well, the AP says. The Cuban government has denied involvement, and no “piece of equipment” that might be causing the symptoms has been discovered yet, State Department spokesperson Heather Nauert told reporters.

That could be because a weapon that covertly uses sound energy to injure people doesn’t actually exist, experts say. “It sounds very appealing and interesting, but I find it hard to believe that there actually is such a device,” says hearing expert John Oghalai, the chair of head and neck surgery at the University of Southern California.

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The weapons known to use sound — like Flash Bang grenades or the sound cannons used against protesters, for example — are loud, obvious, and have immediate effects. No one could describe these noisy devices as covert. “Obviously, we don’t know what any of the investigators have in terms of narrowing it down to say that it’s an acoustic weapon,” says James Jauchem, a retired scientist who previously investigated the biological effects of acoustic energy for the Air Force Research Laboratory. “I’d be highly skeptical of the reports.”

Early reports blamed a sound weapon for the attacks — but based on what we know, it seems unlikely that a sound weapon was used. Nauert told reporters that people including US government employees began experiencing “some kind of symptoms” starting in December 2016. The most recent reported incident, she said, was in late August. Nauert has not confirmed what the specific symptoms are, or said more about the “incidents” that caused them, beyond calling the situation “dangerous.”

News reports are a little more detailed: some of these incidents might have been attacks with a covert sound weapon that “operated outside the range of audible sound and had been deployed either inside or outside their residences,” the Associated Press claimed, based on the statements of unnamed US officials. Other attacks may have produced a loud buzzing or scraping noise, anonymous government sources told CNN. On Thursday, the AP added that these incidents may have happened at night, when victims felt vibrations or heard ringing noises near their beds at home, and, reportedly, at a hotel. However, others who experienced symptoms don’t remember hearing or feeling anything out of the ordinary, the AP says.

Silence would be unusual for a weapon that uses sound energy to disorient, incapacitate, or deter people. In theory, a sonic weapon could do this by causing ear pain, by making a person dizzy, or by vibrating a person’s insides at a frequency that could “stun them, nauseate them, ‘or even liquefy their bowels and reduce them to quivering diarrheic messes,'” as one journalist wrote in the 1990s.

In reality, sonic weapons are much less sophisticated than the hype, and they involve significantly less diarrhea. They work by being noisy and obnoxious — which means that the crudest sonic weapon is loud music. In 1989, for instance, US forces unleashed a barrage of Black Sabbath and Guns N’ Roses to drive Panamanian dictator Manuel Noriega out of hiding. In 1993, the FBI tried the same trick — blasting recordings of dying rabbits and Tibetan prayer chants in an effort to end a standoff with the Texas-based religious group, the Branch Davidians. (The attempt didn’t work.) Loud music has also been used to torture detainees at Guantanamo.

The Department of Defense’s list of non-lethal weapons includes ones specifically designed to use sound — but these work by loudly startling their targets, not by subtly causing major health complications. The flash bang grenade, for example, uses a bright flash of light and a loud bang to stun its victims.

The list also includes so-called acoustic hailing devices that can project both verbal commands and earsplitting beams of noise through the air (or underwater). One such device is called the Long Range Acoustic Device or LRAD, a sound cannon that can “send messages, warnings, and harmful, pain-inducing tones over longer distances than normal loudspeakers.” If fired too close to its targets, the LRAD can cause ear pain and long term hearing loss, according to the 2009 book ‘Non-Lethal’ Weapons. The New York Police Department faced a lawsuit for unleashing the LRAD on protesters and journalists in 2014.

Of course, the potential to cause deafness is a major design flaw for a sound weapon. Why? “[I]t can be expected that the weapon will cease to work if the victim quickly becomes permanently deaf from exposure to high intensity sound,” according to a report prepared for the Space and Naval Warfare Systems Center.

There are certain sounds that humans just can’t hear. Dog whistles, which are too high pitched for the human ear, are in the ultrasonic range. The rumbles of earthquakes, which are too low pitched for us to hear, fall in the infrasonic range. But for a sound weapon to cause hearing loss, “You have to actually hear it,” Oghalai says.

Hearing loss is only one of the symptoms that the diplomats are reportedly experiencing — could ultrasound or infrasound cause dizziness, nausea, or brain damage? “There’s not that much evidence there at those frequency ranges about how it impacts human health,” says Nandini Iyer, a research audiologist with the Air Force Research Laboratory. “We’re just as intrigued as you are,” she adds.

To be clear, ultrasound can affect the human body — but only if the device generating those ultrasonic waves is pressed right up against it. Doctors use ultrasound for medical imaging, for example. And ultrasonic sound waves can heat up tissue, according to that Space and Naval Warfare Systems Center report. However, the report adds, “In air and water, the power of ultrasound rapidly falls off with distance.”

“It would be difficult to come up with any way to produce a weapon of that kind,” Jauchem says. Not even rodents are bothered by pest control devices that promise to scare animals away with irritating ultrasonic noise — and unlike us, mice can actually hear those high pitched sounds. So an ultrasonic weapon that works against people is even less likely.

Jauchem’s research focused on the other end of the spectrum: infrasound. His own experiments showed that two minipigs weren’t put off their cracked corn during stints in a chamber resonating with infrasonic waves. Monkeys weren’t bothered by it either. After digging through decades of literature in the field, Jauchem concluded that infrasonic weapons are impractical.

While there’s speculation that infrasound might be able to vibrate the body’s organs or inner ear to cause pain, vertigo, and nausea, “These are things that haven’t been proven,” Jauchem says. “There’s a lot of overhype in terms of the possible effects.”

A much more likely explanation for the symptoms is some kind of chemical exposure, Oghalai says. “That’s a lot more simple.”

Chemicals known to damage hearing include heavy metals like mercury and lead, as well as some industrial solvents used to make rubbers and plastics, according to the American College of Occupational and Environmental Medicine’s task force on occupational hearing loss.

Illnesses can also cause sudden hearing loss. Respiratory and ear infections, for example, can sometimes lead to inner ear inflammation called labyrinthitis. The symptoms of labyrinthitis should sound familiar: vertigo, hearing loss, bad balance, nausea, and ringing in the ears — all symptoms the diplomats are said to have experienced. The measles, mumps, West Nile, and Zika viruses have been linked to some degree of deafness — as have medications, like certain antibiotics, aspirin in excessive amounts, and some diuretics.

It certainly would be wild for the existence of an extraordinary new sonic weapon to be discovered, but so far it seems improbable. It’s hard to jump to the strangest and most fantastical conclusion when there may be other plausible explanations out there. But for now, scientists still have a big mystery to solve.