So far, 21 Americans have reported symptoms, and Canadian diplomats are suffering as well. It's gotten so bad that the United States decided this week to yank all nonessential personnel from its Havana embassy. Americans are being warned against visiting the country for their own safety until investigators can figure out what's going on.

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What is going on? For months, experts have struggled to explain what kind of weapon could cause such a wide variety of symptoms. Investigators on the scene have uncovered few clues. In the absence of hard proof, there are lots and lots of theories. Here are some of the main ones:

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The perpetrators are using sound as a weapon

The sonic attack theory is a popular one, especially because some of the diplomats are reporting hearing loss, sounds and vibrations.

It is possible to use sound waves to cause problems. Ultrasonic frequencies, which are high-pitched, can be harnessed and directed. As Tim Leighton, professor of ultrasonics and underwater acoustics at the University of Southampton, told the Guardian: “If you want to produce a tight beam of energy that you can point at someone, ultrasound is the one to go for.” Studies have shown prolonged exposure to ultrasonic sound can result in hearing loss and human tissue damage.

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It's also not hard, professors say, to build a device that emits this kind of noise. “You can buy transducers on the Internet that emit these frequencies,” Robin Cleveland, a professor of engineering science at the University of Oxford, told the Guardian. “Anybody with a bit of engineering background could put one together.” It would take a device about a size of a matchbox to produce noise that could, at close range, induce feelings of anxiety or difficulty concentrating.

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High frequency sound doesn't travel well through any kind of barrier, like a wall or even a curtain. It's even hard for it to pass through human skin. To create a sound that could travel through windows, you'd need something more like the size of a suitcase. To affect people 150 feet away, the device would have to be the size of a car.

Scientists are also skeptical about ultrasonic sound's potential to cause permanent brain damage. (According to U.S. officials, some Cuban diplomats had been diagnosed with mild traumatic brain injury.) “That’s a little harder for me to believe,” Cleveland told the Guardian. “The sound would have to enter the brain tissue itself, but if you’ve ever had an ultrasound scan you’ll know they put gel on. If there’s even a tiny bit of air between the sound and your body it doesn’t get through.”

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In short: Weaponizing sound is a glamorous theory, but experts said they don't think that's what's going here. “It sounds very appealing and interesting, but I find it hard to believe that there actually is such a device,” hearing expert John Oghalai, who chairs the Caruso Department of Otolaryngology at the University of Southern California, told the Verge.

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Okay. So, maybe it's an electromagnetic device?

The case for: Electromagnetic waves can be easily directed, like a laser. They can also travel through walls and could plausibly be concealed from afar. (In the 1960s, the Soviet Union bombarded the U.S. Embassy in Moscow with microwaves; it's not clear why or whether that had any impact.) Electromagnetic pulses, when sent out in short, intense blasts, can also cause people to “hear” clicking sounds.

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Electromagnetic waves usually cause physical damage by heating body tissue. The diplomats haven't reported burning sensations.

So maybe it's a poisoning? Could it be a chemical weapon?

Yes. There are several chemicals that can cause hearing damage, including mercury and lead, along with some industrial solvents.

What about the other symptoms?

Writing in USA Today, Director of Medicine at the American Council on Science and Health Jamie Wells and microbiologist Alex Berezow explain it's possible, particularly “if the diplomats share meals together, it is a distinct possibility that somebody poisoned their food.”

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“Does chemical poisoning explain all the known symptoms, even for those victims who heard noises in the middle of the night?” they write. “Possibly. Chemical solvents can cause nerve damage, which can manifest in different ways. With auditory nerve damage, some people might experience ringing (tinnitus), and others might find certain noise frequencies excruciatingly intolerable while others barely notice.”

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Or maybe the diplomats just got sick?

Respiratory and ear infections can sometimes cause hearing loss. One inner-ear inflammation called labyrinthitis can lead to vertigo, hearing loss, bad balance, nausea and ringing in the ears — all symptoms the diplomats experienced. Of course, the victims have been tested for the obvious diseases, but maybe they're suffering some kind of new or mutated illness doctors don't know to look for yet.

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One reason to be skeptical: Though American diplomats work closely with Cuban staff at the embassy, only Americans got sick. If the victims were suffering from a contagious disease, you'd expect it to have spread more widely.

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Is Cuba to blame?

We don't know for sure, obviously.

Experts say the Cuban government has been working closely with the United States to figure out what's going on. The Cuban president met with the top U.S. envoy in the country to express his grave concern and confusion about what's going on. Cuban officials even let the FBI come down to Havana to investigate, an extraordinary level of access. (Also, Cuba has no obvious beef with Canada.)

Some U.S. officials are still skeptical. Investigators have begun to wonder whether this is the work of a rogue faction of Cuba’s security forces. Or maybe it's another country, like Russia or North Korea. Perhaps Moscow is trying to drive a wedge between communist Cuba and the West? (As the AP reports, “Russia also has advanced, hard-to-detect weaponry that much of the world lacks and might not even know about.")

Or, most unsatisfying: Maybe it's no one at all? It's possible the diplomats were exposed accidentally to the chemical that's now wreaking havoc. Or maybe the culprit is testing out some new surveillance system that's gone awry?