On January 23, 1973, the 5,000 residents of the tiny Icelandic island of Heimaey—located in the Westman archipelago just four miles off Iceland’s southern coast—woke up to the Earth splitting open. In the dead of night, a mile-long volcanic fissure ripped through the ground just a half-mile fr om the town of Vestmannaeyjar, marking the birth of a new volcano named Eldfell. Fountains of molten basaltic lava shot hundreds of feet into the sky, raining ash and glowing, golf-ball-sized rocks onto rooftops. Within hours, a makeshift fleet of local fishing boats evacuated almost the entire population to the mainland. The people were safe, but their town faced an existential threat. The threat went far beyond a local crisis. Not only were the townspeople of Vestmannaeyjar completely reliant on the harbor, but all of Iceland depended on this tiny island’s fishing industry for a massive chunk of its exports. If the advancing lava choked off that narrow bay, it would deal a devastating blow to the entire nation’s economy. The Eldfell volcano on the tiny Icelandic island of Heimaey began erupting on January 23, 1973. Image: Contributor / Getty Images / Bettmann Freezing a boiling hot flow of lava As the lava advanced, Icelandic physicist Thorbjörn Sigurgeirsson proposed a radical idea. What if they just froze it? It sounded crazy, but basaltic lava offered a tiny loophole. While it erupted at a scorching 2,000 degrees Fahrenheit, it crawled forward relatively slowly. Sigurgeirsson hoped that if islanders sprayed enough cold seawater onto the leading edge, they could crust it over and stop its advancement. Standing in the path of an eruption, though, is a daunting physics problem. The frozen artificial shell would act as a natural dam, forcing the liquid fire churning behind it to stall, stack up, or divert away from the harbor. The lava wasn’t just hot; it was a wall of rock weighing millions of tons that would bulldoze standard barriers like toothpicks. So they improvised. Local firemen dragged hoses to the edge of the crunching, glowing mass. They started spraying ice cold Atlantic ocean water on February 7—two weeks after the eruption. The heat was so intense it singed their eyebrows, and the water instantly flashed into blinding clouds of steam and toxic gas. But it worked. The edge of the flow was slowed. Realizing they had proof of concept, the Icelandic government went all in, urgently requesting heavy-duty pumps from the United States. On March 26, 1973, the first shipment of 32 high-capacity pumps touched down. More than two months after the Eldfell volcano began erupting, the United States sent 32 high-capacity water pumps to fight the blaze. Image: Contributor / Getty Images / Fred Ihrt Before the pumps arrived, the islanders’ local gear could only manage 100 liters of water per second—not nearly enough pressure to reach the top of the steep, 100-foot-tall lava walls. Powered by the American diesel pumps, the new system unleashed a torrent of 1,000 liters of water per second through miles of heavy piping and hoses, carrying seawater all the way up the steep slopes to douse the top of the flow. As the lava moved forward, its outer layers cooled and stacked up into an enormous wall of solidifying rock, driven forward by the liquid fire churning inside. Because standard barriers would crumble under that massive weight, cooling the outer shell with seawater was the only way to build a wall strong enough to stop it. Deployed right at Básakers Wharf, the massive American diesel engines vastly increased their output. The “Suicide Squad” takes the field By March, the rescue effort had turned into a surreal, industrial battlefield. Teams of local volunteers, who grimly nicknamed themselves the “Suicide Squad,” took on the most terrifying job. They hiked directly onto the moving crust of the active Eldfell flow to lay miles of plastic pipeline, extending the water system right to the edge of the molten rock where fire hoses could douse the advancing front. The scene was apocalyptic. The crust they stood on was often only a few inches thick. If someone stood still for too long, the rubber soles of their boots melted. If water stopped flowing through a pipe for even a second, the trapped water would flash-boil and explode. Surrounded by choking sulfur gas and whiteout steam, the Suicide Squad and engineering crews pumped about 7.3 billion gallons of seawater over five months. It was a filthy, grueling slog, but the makeshift barrier held.  Local volunteers hiked directly onto the moving crust of the active Eldfell flow to lay miles of plastic pipeline, extending the water system right to the edge of the molten rock where fire hoses could douse the advancing front. Image: Contributor / Getty Images / Fred Ihrt By July, over 400 homes lay buried under ash, but the 100-foot lava wall didn’t crumble. It froze solid. That massive cliff of cooled rock permanently altered the coast, creating a new breakwater that actually left the harbor safer than before. Amazingly, not a single life was lost. Redefining volcanic risk mitigation Before Heimaey, the rule for dealing with volcanoes was a simple one: run away. Vestmannaeyjar’s 1973 victory reshaped global volcanic risk mitigation. “In some ways, the success of this response has set unrealistic expectations for stopping lava flows,” says Benjamin Andrews, a geologist at the Smithsonian’s National Museum of Natural History.  “There is also a misconception that the lava was ‘stopped,’ when it was diverted—this is a big deal when we think about redirecting lava flows. The lava needs to go somewhere.” Instead of magically freezing the volcano in its tracks, they used the cooled rock to build a dam that redirected the lava away from the harbor entrance—forcing the flow to spill harmlessly out into the open ocean and extend the island’s eastern shore. By July, over 400 homes on the island lay buried in ash, but firefighters and locals had saved the harbor. Image: Contributor / Getty Images / Fred Ihrt When it comes to current volcanic defense, Andrews notes that modern efforts have shifted toward physical engineering to safeguard key facilities such as the Svartsengi Power Plant near Grindavík.  “A more effective strategy is to build barriers around critical infrastructure, such as the earthen berms that the Icelanders have built around some of the power plants on the Reykjanes Peninsula. These barriers have diverted lava flows from critical facilities,” says Andrews. In a twist of geological irony, Eldfell’s cooled barrier actually extended the harbor’s mouth, leaving it more protected from fierce Atlantic storms than it was before the disaster. It proved that with a lucky mix of geography, infinite water, and stubborn human grit, we don’t have to just stand by and watch everything turn to ash when a volcano decides to erupt. 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