Wonder Woman begins on the paradisiacal island home of the Amazons, a mythical tribe of warrior women sent to Earth to protect humans from their basest impulses. When World War I spills onto the island’s shores, the film’s titular heroine, Diana (Gal Gadot), leaves its relative safety to help American spy Steve Trevor (Chris Pine) deliver a German laboratory notebook to British intelligence. That notebook, stolen from a German chemist nicknamed Dr. Poison, contains the Germans’ newest superweapon: “It’s a formula for a new kind of gas, mustard gas,” Diana explains, translating the notebook for Britain’s military leaders. “Hydrogen-based instead of sulfur.”

“Gas masks would be useless against hydrogen!” gasps one of those leaders. Even in a film that includes a magical island created by Zeus himself, these details about the gas are so strangely specific that they deserve a little fact-checking. Chemically speaking, does this formula make any sense?

Not really. First off, mustard gas is such a horrible, terrifying weapon, it doesn’t need to be made more potent. But if you were a chemist bent on raining destruction on the Allied forces, you wouldn’t do it by replacing the sulfur atom in mustard gas with a hydrogen atom. You’d know that sulfur is the linchpin holding together this poisonous molecule.

“It’s got a bat-like structure,” explains Raychelle Burks, a chemistry professor at St. Edward’s University in Texas. “Sulfur’s smack in the middle, and then you’ve got two bat wings that come out — and they’re kind of crinkly cut.” Both of those bat wings are made up of two carbon atoms and capped with a chlorine. The four carbon atoms each sprout two little tufts of hydrogen fuzz, for a total of eight hydrogen atoms.

“Although there is a villain in a human form, the race against time is a chemistry one.”

So, mustard gas — also known as sulfur mustard — has plenty of hydrogen already. And an additional hydrogen couldn’t replace sulfur, because hydrogen only likes to bond to one other atom at a time. Unlike sulfur, hydrogen doesn’t have enough spare hands to hold on to both bat wings.

Still, Burks says, the film does get the prominent role of chemistry in WWI right. “Although there is a villain in a human form, the race against time is a chemistry one,” she says. “In that way, they did capture World War I as being a chemist’s war.”

The real-life Dr. Poison was a German chemist named Fritz Haber. He won the Nobel Prize for the breakthrough that made the large-scale production of fertilizer possible. He also spearheaded Germany’s chemical weapons program, and presided over the first successful use of a chemical weapon during World War I: not mustard gas, but chlorine. With the help of the wind, the Germans wafted chlorine gas across the Allies’ trenches on a spring day in 1915 and killed more than 1,100 soldiers, writes Sarah Everts in Chemical & Engineering News.

This experiment in chemical warfare sparked an arms race that earned World War I the moniker “The Chemist’s War,” despite an international ban on chemical weapons. Everts quotes German officer Rudolf Binding, who wrote: “I am not pleased with the idea of poisoning men. Of course, the entire world will rage about it at first and then imitate us.”

Each new chemical agent was met with a more sophisticated countermeasure

Binding was right. A twisted call-and-response ensued — where each new chemical agent was met with a more sophisticated countermeasure. (Among the earliest defenses against chlorine gas were urine-soaked handkerchiefs; the ammonia in urine could render the chlorine relatively harmless.)

But when the Germans first weaponized mustard gas in 1917, it presented a new challenge. Contrary to its name, mustard gas does not actually have any mustard in it. (It’s named for its mustard-like odor.) Nor is it a gas. “Soon known as the ‘king of the battle gases,’ mustard gas is actually an oily liquid that can pass through leather, rubber, and most textiles,” Everts writes.

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Artillery shells filled with mustard gas rained tiny droplets onto the battlefield that didn’t need to be inhaled to inflict mass casualties. They soaked into the soldiers’ uniforms and covered their skin, causing a delayed reaction. Hours after exposure, soldiers would start to vomit, and their skin would burn. Huge blisters would form, and, if they inhaled the mustard gas, the lining of their lungs sloughed off. Many went at least temporarily blind. Both recovery and death were slow and agonizing. And the oil persisted on soldiers’ bodies and in the environment, where mortars could kick up residual mustard gas that had settled onto the soil months before.

Wonder Woman doesn’t show the gruesome effects of the “hydrogen-based” mustard gas on people. Instead, we see it shriveling a gas mask (which isn’t how mustard gas works), cracking glass (it can’t), and exploding in an airplane, which goes up like a chemical-tainted Hindenburg. It seemed like the filmmakers wanted to make the gas more flammable, Burks speculates, and people might equate hydrogen with flames. “Like ‘Can we have a chimera?’ And the answer is ‘No. You cannot.’”

“You don’t need to be the big, scary monster. No, this is scarier than that.”

Still, the film captures the aftermath and horror of chemical weapons in a scene where the Germans bomb a village with artillery shells containing the fictionally souped-up mustard gas. An orange mist floats low over the ground as Diana walks through to find scattered bodies. The thing that caused that devastation? A tiny molecule you can’t fight with your fists, or with weapons. Once that creeping mist catches you, it’s too late. “Bigger is not scarier,” Burks says. “You don’t need to be the big, scary monster. No, this is scarier than that. And it’s a teeny, teeny molecule.”

Wonder Woman is a superhero movie, so it doesn’t give anything away to say that Diana beats the bad guy in the end. But knowing that chemical weapons continue to be used even today touches that victory with a sense of futility, which the filmmakers seem to have intended. After all, humans will continue to dream up new, horrific ways of hurting one another. But Wonder Woman gives us a chance to escape to an imaginary world — where there’s someone or something out there, trying to stop us.

Update June 10th 9:10AM ET: This article was originally published on June 6, 2017 and has been updated to include video.