In the summer of 1831, strange things began to happen in the skies over Europe and North America. The sun shimmered blue and green. A veil of haze settled over cities. Crops failed, rainfall patterns warped, and then came the cold.

For nearly two centuries, the cause of this atmospheric disarray remained a mystery. Scientists suspected a volcano had erupted somewhere in the world with enough force to hurl sulfur high into the stratosphere—cooling the Northern Hemisphere by almost a full degree Celsius. But the volcano itself, its name and place, remained stubbornly out of reach.

Until now.

The Incognito Volcano

It turns out the answer had been hiding in plain sight—on a remote, uninhabited island once used to dock Soviet nuclear submarines.

Simushir Island, a sliver of land in the Kuril archipelago stretching between Japan and Russia, is home to a chain of four volcanoes lined up like sentinels along its spine: Milna, Zavaritski, Prevo, and Uratman. None have erupted since the mid-20th century. But one of them, Zavaritski, is the new candidate for the source of the great mystery eruption of 1831.

“We had no idea this volcano was responsible [for the dramatic eruption],” said William Hutchison, a volcanologist at the University of St Andrews and lead author of the new study, told Live Science. “It was completely off the radar.” The study was published in PNAS in December 2024.

That changed when Hutchison and his colleagues matched microscopic shards of volcanic glass—cryptotephra—found in Greenland ice cores to ash deposits sampled from the Zavaritski caldera. The chemical signatures were identical.

“The moment in the lab when we analysed the two ashes together, one from the volcano and one from the ice core, was a genuine eureka moment,” said Hutchison. “I couldn’t believe the numbers were identical.”

The Science of Shadows

Large eruptions like this one punch sulfur dioxide into the stratosphere, where it forms sulfate aerosols that reflect sunlight and cool the ground below. The 1831 eruption injected an estimated 12 ± 3.5 teragrams of sulfur into the atmosphere—more than the 1991 Mount Pinatubo eruption, which caused global cooling of about 0.6°C.

Using geochemical analysis, the researchers confirmed the ice-core sulfur wasn’t from a shallow volcanic source like the Mediterranean island of Ferdinandea, previously considered a candidate. Unlike Ferdinandea’s sulfur, which would have carried a distinct isotopic fingerprint from its interaction with marine rocks, the ice-core samples bore no such signs.

Instead, the fingerprints pointed north—to the frigid, windswept reaches of the Kuril Islands, and to the glassy volcanic shards of Zavaritski.

This cryptic eruption had left almost no historical records. Simushir was sparsely populated in the 1800s—inhabited only by small settlements of Ainu, Russian colonists, and conscripted Aleuts. The island’s isolation meant no one witnessed the event—or if they did, their accounts never reached the outside world.

From Ash to Impact

The eruption, dubbed Zav-1, was a powerful one. The team used radiocarbon dating, tephra thickness modeling, and comparisons with other volcanic events to estimate its magnitude between 5.5 and 6. That puts it in the same class as the eruption of Cosegüina in 1835 and Mount Pinatubo in 1991.

The blast carved out a nested caldera visible to this day. Some of its volcanic material—fine pumice and ash—reached as far as the neighboring islands of Chirpoi and Urup. But its most dramatic influence took place in the skies, thousands of kilometers away.

In August 1831, observers across the Northern Hemisphere reported unusually colored suns and moons, Bishop’s rings, and ash “falling from the sky.” While some of these sightings were likely caused by the smaller Ferdinandea eruption closer to Europe, the prolonged climatic disruptions—including famines in India and Japan—line up with the long-lived aerosol veil from Zavaritski.

A Window Into the Past—and Future

The revelation has important implications beyond filling in a historical blank. By pinpointing Zavaritski as the source, scientists can now refine climate models that simulate volcanic forcing—a key factor in understanding both past climate shifts and the future risks posed by large eruptions.

It also underscores how much remains unknown about Earth’s volcanic systems. The Kuril Islands, riddled with active volcanoes and prone to seismic unrest, remain one of the least monitored regions in the Ring of Fire. The discovery of Zavaritski’s role in shaping 19th-century climate invites a sobering question: what other climate-altering eruptions might still be hidden in the geological record?