Thunder, lightning and flickering candles. It sounds like the stuff of a horror story—and for Mary Shelley, it was. She wrote her masterpiece Frankenstein when she was just 19 years old, and the dark, stormy summer nights that helped bring her monstrous creation to life were nearly as dramatic as the novel itself.

Strangely enough, the saga of Frankenstein started not with a vision but with a volcano. In 1815, a gigantic volcanic eruption at Mount Tambora in Indonesia choked the air with ash and dust. The eruption killed roughly 100,000 people in its immediate aftermath, but the overall toll ended up being much higher—it is now considered to be the deadliest volcano eruption in history.

The next summer, the warm growing season never came. Instead of sunshine, most of Europe was covered in fog and even frost. Crop failures stretched across Europe, Asia and even North America for three years afterward. Famines, epidemics and political revolts followed. Historians estimate that at least a million people starved in the aftermath of Tambora’s eruption, while tens of millions died from a global cholera pandemic that it unleashed.

During those three years of darkness and famine, some of Europe’s greatest artists created their darkest and most enduring works. Mary Shelley was among them—but when she arrived at Lake Geneva in May 1816, she was looking for a vacation, not literary inspiration. Unfortunately, the weather was so ghastly in Switzerland that she was trapped inside nearly the entire time.

Mary traveled with her lover, poet Percy Bysshe Shelley, their four-month-old baby and her stepsister, Claire Clairmont. At the time, Claire was pregnant with a child by Lord Byron, the groundbreaking poet whose personal affairs had made him one of England’s most divisive celebrities. Most recently he had divorced his wife and, rumor had it, continued an affair with his half-sister. Plagued by gossip and debt, he decided to leave Europe.

After Byron’s departure, the obsessed Claire convinced Mary and Percy to travel to Geneva with her. A few days later, Byron—clearly unaware that Claire would be there—arrived in town. Mary, who had eloped with her married husband when she was just 17 and was subsequently disowned by her intellectual family, sympathized with the scandalous poet.

Percy and Byron, who had been fans of one another’s work, soon formed an intense friendship. They abandoned their other travel plans and rented nearby properties along Lake Geneva. During the frigid evenings, they gathered with the rest of the group at the Villa Diodati, the stately mansion Byron had rented for his stay along with John Polidori, his doctor. They read poetry, argued, and talked late into the night.

The terrible weather kept them inside more often than not. Thunder and lightning echoed through the villa and their conversations turned to one of the big debates of the day: whether human corpses could be galvanized, or re-animated, after death. Mary, who described herself as “a devout but nearly silent listener,” sat near the men and absorbed every word of their speculation about the limits of modern medicine.

As the days plodded on, conflicts between the vacationers began to simmer. Byron was annoyed by Claire’s attempts to enchant him. Mary had to fight off sexual advances from Polidori, who had become obsessed with her. Percy was depressed. By the time three days of rain trapped them inside the villa, tensions had reached a boiling point.

They coped by reading horror stories and morbid poems. One night, as they sat in the candlelit darkness, Byron gave them all a challenge: write a ghost story that was better than the ones they had just read. Inspired by a tale of Byron’s, Polidori immediately complied. His novella “The Vampyre,” published in 1819, is the first work of fiction to include a blood-sucking hero—which many think was modeled on Byron himself.

Mary wanted to write a story, too, but she couldn’t land on a subject. “I was asked each morning, and each morning I was forced to reply with a mortifying negative,” she later wrote. But one sleepless night, as thunder and lightning echoed off the lake, she had a vision. “I saw the hideous phantasm of a man stretched out,” she wrote, “and then, on the working of some powerful engine, show signs of life.”

The next morning, she could say yes when she was asked if she had a ghost story in mind. Her book, Frankenstein, or the Modern Prometheus, incorporated the eerie setting of the Villa Diodati and the morbid conversations of the poets. The story she later called her “hideous progeny” asks what happens when men pretend they are gods—inspired, perhaps, by the hubris of the company she kept in Switzerland.

Though she did not know it, Mary’s book, which was published in 1818, would go on to revolutionize literature and popular culture. But the lives of the vacationers did not end happily. Polidori committed suicide in 1821. Percy Shelley drowned during a freak storm in 1822, when he was just 29 years old. Byron took the daughter he had with Claire, Allegra, away from her mother and sent her to a convent to be educated; she died there in 1822 at age 5. Byron died in 1824 after contracting a fever.

Of the group, only Mary and Claire lived past age 50. But the book that creepy summer inspired—and its terrifying story of life after death—lives on today.