On the evening of August 8, 1991, after only a few hours of warning tremors, Hudson tore open. The first phase was phreatomagmatic, magma meeting ice and water, ripping a four-kilometer fissure across the northwestern caldera. Four days later a second, more violent vent opened in the southwest and drove a Plinian column twelve kilometers into the sky. More than four cubic kilometers of ash fell across roughly 150,000 square kilometers of Chile and Argentina. Then the winds carried it further than anyone expected. The plume circled Antarctica on the polar westerlies, and ash settled on snow at the South Pole that December. Pilots reported the cloud as far as Melbourne. Particles from this single Patagonian eruption have since been found frozen into the ice of Mount Everest. It remains the second-largest historic eruption in Chilean history, behind only Quizapu in 1932.

The numbers are abstract until you imagine the people and animals beneath them. The ash buried pastures and contaminated water across the sheep country of southern Patagonia. Across Argentina's Santa Cruz Province, where damage ran past ten million dollars, roughly half of all grazing animals died, their fleeces weighed down with grit, their food poisoned. Shepherds and farmers reported their breathing and eyesight failing in the irritating dust. House roofs collapsed under the load. For families already enduring a hard winter and a fragile economy, the eruption was a final blow that emptied parts of the region, driving a depopulation the Chilean state had to soften with aid. These were not statistics. They were ranching families on remote estancias, watching a year's livelihood smother under gray powder that drifted for weeks after the volcano itself had gone quiet.

Hudson sits at one of the most geologically strange places on the planet. Just offshore, an entire oceanic ridge is diving into the trench beneath South America, forming the Chile Triple Junction, and the descending plate has torn open a window in the slab below. That window helps explain why Hudson is so unusually active: it sits just east of the junction, fed by hot young crust and a tangle of faults. South of the volcano, the chain of fire simply stops. A 350-kilometer gap, empty of active volcanoes, separates Hudson from the next eruptive zone, and the volcano marks the near-southern limit of the Andes' Southern Volcanic Zone. It is often, if incorrectly, called the southernmost of its kind.

The 1991 eruption was not even Hudson's worst. Around 7,750 years ago, the volcano produced the largest known Holocene eruption in the southern Andes, an event so vast its ash blanketed all of southern Patagonia and more than 40,000 square kilometers of Tierra del Fuego, in places lying meters thick. Researchers believe that eruption may have devastated the hunter-gatherer populations of Tierra del Fuego, possibly wiping out human life on the island entirely before it was resettled generations later by people arriving in bark canoes. Hudson has not finished. It stirred again in 2011, prompting the evacuation of about 140 people, and roughly 84,000 now live within fifty kilometers of a mountain that hides its power beneath the ice and waits.