Something very strange is happening to America’s diplomats.

In late 2016 through early 2017, at least 24 US diplomats in Cuba — and several Canadian diplomats — complained of hearing loss, nausea, loss of balance, and headaches.

In late November 2017 through April 2018, at least two US officials in Guangzhou, China, complained of similar symptoms.

And on Friday, the US removed another two workers from the embassy in Cuba and checking to see if they also suffered brain injuries.

Some of the affected diplomats and other employees now have mild brain damage and blood disorders, and two may have completely and permanently lost their hearing. A few had to return to the US because of their injuries. Canada went as far as pulling all families of diplomatic staff out of Cuba, just to be safe.

Here’s the strangest thing: Nobody seems to know what could be causing all of this. Experts — from top doctors to US law enforcement officials — have looked into the issue, and they’re all flummoxed.

Could it be some kind of previously unknown sonic weapon? Is someone waging a shadow war against American and Canadian diplomats abroad? Is it some kind of poison? Or, maybe, is it some form of mass hysteria, similar to when people danced themselves to death in the 1500s? (Yes, that’s a real thing.)

It’s gotten so bad that Secretary of State Mike Pompeo has now convened a task force to figure out what’s going on.

On Tuesday, Pompeo said the new group will serve as a “coordinating body for department and interagency activities, including identification and treatment of affected personnel and family members, investigation and risk mitigation, messaging, and diplomatic outreach.”

China has promised to help the United States figure out the cause of the mysterious symptoms. During a press conference alongside Pompeo in May, Chinese Foreign Minister Wang Yi said China has found no instances of “sonic influence.” The Cuban government denies any involvement in the supposed attack.

Even doctors don’t know what’s happening. In a March study by medical investigators who looked at the patients’ symptoms, they said the cause of the trauma “remains elusive” but raises “concern about a new mechanism for possible acquired brain injury from an exposure of unknown origin.”

So what is going on? Here are some theories.

What’s happening to US diplomats?

1) Sonic weapon

The first and most obvious one is that there is some kind of sonic weapon being used to target the diplomats, although it’s unclear if Cuba or China has that kind of weaponry, even though sonic devices exist.

For example, there’s a “mosquito” that produces a high-pitched sound and long-range acoustic devices that police sometimes use to disperse crowds.

But there’s a big problem with this theory, says Sharon Weinberger, an expert on weapons technology and author of the book The Imagineers of War: The kind of weapon that does what the victims describe would defy the laws of physics.

Plus, she added, why would a country choose this kind of weapon to harm a US official, when simply getting someone to beat them up would be easier?

“The danger of saying it’s a sonic weapon is that you’re focusing on the least likely explanation,” Weinberger told me.

2) Sonic listening device

US diplomats could be the victims of a sonic listening device that emits harmful waves. A sonic device used for spying — and wielded by Cuba, China, or even another country like Russia — could unintentionally cause brain damage.

These devices usually operate at the ultrasonic level (above audible range) or the infrasonic level (below audible range). Some doctors believe infrasound is more likely to have been used because it travels greater distances than ultrasound, so attackers could use it from farther away.

US officials said some of the possible attacks in Cuba were audible and made a “deafeningly loud sound, similar to the buzzing created by insects or metal scraping across a floor.”

Infrasound can also cause some of the effects the American and Canadian diplomats experienced. For example, a November 2001 study by the National Institutes of Health noted that one of the symptoms of exposure to infrasound is hearing loss.

And listening devices have been used against diplomats before.

From 1953 to 1976, the US Embassy in Moscow was subjected to microwave radiation from the Soviets as they tried to listen in on conversations. But according to a government study, there was “no convincing evidence” that the microwaves caused any adverse health effects.

3) Toxin

The New York Times notes that a toxin could be causing the brain injuries and hearing problems.

But Mitchell Valdes-Sosa, an expert in auditory physiology from Cuba, told Scientific American in February that poisoning or infectious disease is unlikely. Part of the reason is that diseases in the area — like dengue or Zika — wouldn’t produce concussion-like symptoms as described by the potential victims. Further, it would be hard to target all these separate individuals with such a precise poisoning agent.

4) Mass hysteria

And for mass hysteria, it would mean other US diplomats in China feared so much about what happened to their colleagues that they’ve effectively worried themselves sick.

It’s unclear if victims told each other what they were experiencing, and if that made others feel that they had similar symptoms.

Stranger things have happened. Here’s one particularly odd case of mass hysteria: In 1518, Frau Troffea started dancing — without any music — in Strasbourg. She kept dancing for six days. After a week, 34 people joined her; after a month, 400 dancers were participating. According to the BBC, “15 residents were dying each day from strokes, heart attacks, and sheer exhaustion.”

That said, medical investigators wrote in their March report that some of the diplomats who had complained of symptoms did lose some of their hearing — meaning the harm was serious and, in fact, real.

The hope now is that someone can solve this odd mystery before even more US officials get hurt.