In other ways, however, Flammarion lacked Sagan’s media savvy and even common sense, as the Herald article of late November 1909 demonstrates. Partway through, Flammarion muses about what would happen when the planet drifted through the comet’s tail. He prefaces this discussion by pointing out that the tail was sparse—far sparser than a cloud—and the consequences of Earth’s passage through it would therefore almost certainly be nil.

Those caveats out of the way, Flammarion proceeds to speculate wildly. Perhaps, he writes, the hydrogen in the comet’s tail will combine with the oxygen in our atmosphere and strip out every molecule we need to breathe, leaving us choking to death. He offers no plausible mechanism for this reaction—he just throws the idea out there. Similarly, he imagines carbonic acid in the tail searing our lungs, or reactions that could trigger a “diminution of nitrogen and an excess of oxygen” and extinguish “the human race … in a paroxysm of joy and delirium, probably delighted at their fate.”

Flammarion finishes the articles by reassuring readers that, no matter what perils lay ahead, the cometary tail would be akin to “a fog through which a locomotive was dashing at full speed.” But when other papers picked up the Herald story, guess which part people latched onto—the sober reassurance that we need not worry, or the cinematic horrors of mass death from the skies?

Flammarion was not the only one making irresponsible claims. Another French astronomer speculated, also with no supporting evidence, that the tail might somehow produce a barrage of X-rays and cause all the water vapor in the atmosphere to condense at once, triggering the biggest flood the world has seen since Noah. In the public imagination, the fascination with the comet was now drenched with fear.

Things didn’t get out of hand, however, until February 1910. Teams of astronomers at the Yerkes Observatory in Wisconsin had been training more than a dozen telescopes on the comet for weeks. An observer compared the array to artillery guns on a battleship, and cots and blankets lay nearby for researchers to catch naps when necessary. On February 7, the Yerkes scientists announced a modest discovery: using spectrograph analysis, they had detected cyanogen gas in Halley’s tail.

Cyanogen (C₂N₂) is related to cyanide, and like its more famous cousin, it has a pungent almond odor and kills by preventing cells from utilizing oxygen. After hearing Earth might be streaking toward a cloud of toxic gas, a New York Times reporter called Flammarion for his take on the matter.

The Frenchman again showed an astonishing lack of awareness. Without saying it was likely, Flammarion mused that the cyanogen could theoretically “impregnate the atmosphere and possibly snuff out all life on the planet.” To be sure, other astronomers quoted in the story threw cold water on the idea. But no one paid attention to them, and the Times’ headline sure didn’t help: “Comet’s Poisonous Tail.” Other newspapers once again ran with the story, and the scientists’ proclamations drove many to terror or despair.

Some farmers in Germany declined to plant crops that spring, reasoning that they would die before anything ripened. Creditors began defaulting on bank loans, deciding to live it up while they had time. Snake-oil salesmen peddled “comet pills” as a supposed antidote to celestial toxins. Bartenders promised similar protection: if you had enough scotch or whiskey in your bloodstream, they claimed, cyanogen couldn’t touch you. A broker in Los Angeles began selling “comet insurance,” offering $500 cash to the families of anyone killed by Halley’s passage. How the beneficiaries would actually collect the money in the case of a worldwide apocalypse—or what good money would even do in such circumstances—was never clear, but at least 50 customers paid a quarter a week for the coverage. In wealthier Washington, DC, agents charged a dollar per week.

To be sure, not everyone took the omens so seriously or so cynically. As Earth approached the tail in early May, a Brooklyn socialite threw a “comet party,” purposely inviting 13 people to thumb her nose at fate. Festivities began at 2 a.m., and guests arrived to find skeletons and spiderwebs decorating the walls; like a Halloween gala, everyone wore masks. After a quick game of pin the tail on the comet, they headed up to her roof deck to gaze at their would-be nemesis through the host’s telescope. Clouds, alas, blocked their view, but the fad soon spread to other cities.

The terror, however, was real enough in some quarters. Suicide deaths in at least four countries were attributed to the panic. People also lowered themselves into wells or sealed off their homes and papered over the keyholes to fend off heaven-sent death. And some took refuge in caves, as researchers recently discovered in Puerto Rico.

In 2018, speleologist Ángel Acosta-Colón visited a cave in a secluded region in the island’s southern half. He wasn’t so interested in the cave’s geology—it’s mostly run-of-the-mill limestone studded with stalactites and stalagmites. He was drawn instead to the cave’s ancient rock carvings and charcoal wall drawings of animals, humans, animal-human hybrids, and geometrical figures, which to that point had been undocumented. By analyzing the carbon-14 in the charcoal, Acosta-Colón dated some drawings to at least 400 BCE, meaning that the island’s native Taíno people drew them. Depictions of ships and horses from the island’s colonial era also adorn the walls. There’s even a figure that could be a lion—possibly drawn, Acosta-Colón suggests, by an early African slave. In all, the array of pictures span the history of Puerto Rico.

Including modern history. To reach the deepest parts of the cave, Acosta-Colón had to crawl on his hands and knees through a low passage for a dozen minutes. He finally emerged into a low-ceilinged chamber. While poking around the chamber’s recesses, he noticed an odd picture, quite unlike any of the others.

The drawing consisted of three small steps leading up to a thick block; a Christian cross adorned the top. Acosta-Colón took it for a tomb or mausoleum. Most striking of all, a five-pointed star with a sweeping tail was streaking above the mausoleum—or crashing into it. A caption beneath read, “Recuerdos de Fernando Colón, Mayo 23, 1910”—memories of Fernando Colón, May 23, 1910.

Unsure what to make of the drawing, Acosta-Colón snapped some photographs, then returned his attention to the older art, which he and a colleague eventually described in a paper. But the strange picture of the star and tomb kept itching at his mind, and in 2023 he showed a photograph to a colleague at the University of Puerto Rico, astronomer Abel Méndez.

Méndez declined to visit the cave himself. “I don’t like enclosed spaces,” he says, and the crawl there sounded like torture. But the image impressed him. It clearly depicted a comet, and as an astronomer, he recognized the month and year as a probable reference to Halley’s. After researching the history of the comet panic in Puerto Rico, Méndez suspected that Fernando Colón and possibly others had hidden themselves in the cave to escape a deadly cloud of cyanogen gas. He decided to find out the artist’s identity.

Puerto Rico was in flux in 1910, having recently been handed over from Spain to the United States as a prize of the Spanish-American War. The island was still negotiating its status as an American territory, and the U.S. government conducted a thorough census in 1910 to get a better picture of the Puerto Rican people. Méndez searched these census records and found four people named Fernando Colón living near the cave in 1910.

A few clues helped Méndez narrow down the list. The caption used commas and accents properly, meaning the artist had access to good schooling—and likely American schooling. The U.S. government was emphasizing American-style education then, and the date in the caption, May 23, was written not in the Spanish format (day first, month second) but the American format (month first, day second). Helpfully, the 1910 census asked each citizen whether they could read and write in English and Spanish. Of the four Fernando Colóns, just one answered yes to both.

To Méndez and Acosta-Colón’s surprise, the artist in question, Fernando Colón-Vásquez, was just 15 years old in 1910. But upon reflection, Méndez says, it makes sense: “How obvious it was a kid. They’re prone to writing everywhere.” On a return trip to the chamber, Acosta-Colón found a similar steps-and-tomb drawing signed by Colón-Vásquez’s brother Monserate, hinting that perhaps the entire family visited the cave together.

As more details came into focus, it became clear that this gathering was no lighthearted comet party. The Colón-Vásquez family lived in a small town six miles from the cave, and the trip there would not have been a pleasant country stroll. The trek took at least two hours, Méndez and Acosta-Colón estimate, through an arid landscape of thorny plants. In all likelihood, the family would have dragged food, water, fuel for light, and other supplies through that harsh terrain—a miserable chore that would have become more miserable as they began inching through the cave complex. Only a deep fear of Halley’s Comet, it seems, would compel such a journey.

Whatever the motives behind the brothers’ art, Méndez and Acosta-Colón view the drawings as part of an ancient tradition of Puerto Rican cave art.

“I’m a speleologist, and for me, it’s interesting how we continue to use caves,” Acosta-Colón says. “We think, ‘Oh, only cavepeople used them long ago,’ but we continuously use them in Puerto Rico; it’s part of our history.” He notes that this tradition continues today. People have taken shelter in caves during the devastating hurricanes that have struck the island in recent years.

So far, Méndez and Acosta-Colón have discovered only a small amount of information about Fernando Colón-Vásquez’s life. He served in the military as a young man and later worked for a government insurance fund before dying in 1950 at age 54. Two of his children appear to be alive, and the scientists are currently tracking them down to ask if they know anything more about their father’s time in the subterranean refuge.

Even if nothing more emerges, Colón-Vásquez’s drawing remains a prime example of how interactions between scientists and the public can go wrong: the prestige and imprimatur of science is such that people take even wild speculation at face value sometimes. It’s easy to chuckle over the sillier manifestations of the 1910 panic—comet pills, insurance scams, and the like. But the record left by a frightened teenager also helps Méndez and Acosta-Colón appreciate the “blend of fear, fascination, and artistic expression that such events can provoke.”