In 1816, half a foot of snow fell in New England. That would be completely unremarkable. Except that it was in one day—in June.

That same summer, Mary Shelley spent a chilly vacation holed up indoors—and used the time to write Frankenstein. Crops failed around the world, plunging Thomas Jefferson into serious debt for the rest of his life. Oats became scarce in Germany, making horse travel expensive—and leading to the invention of the bicycle. Struggling farmers in China began raising opium, giving rise to a drug trade that has lasted to modern times. And famine in many areas led to widespread disease, including a cholera outbreak that killed millions.

What was the cause of all this chaos? A year earlier, a volcano erupted in Indonesia.

Larger than Krakatoa, Vesuvius, or Mount St. Helens, Mount Tambora erupted for 2 weeks straight. Around it, nearly 100,000 people died, buried under thick layers of ash like in Pompeii.

Greenhouse-gas emissions from the eruption, which could have warmed the atmosphere, were offset by particulates and sulfur dioxide gas. Ash and dust blocked out the sun temporarily, darkening skies around the world. The sulfur dioxide was longer-lasting, becoming aerosols that reflected the sun’s heat for 3 years!

This turned 1816 into “The Year Without a Summer,” as it was called, with long-term global effects. The good news? The atmosphere recovered within a decade, and life went back to normal.