In 1935, the US Army dropped bombs on Hawaii’s Mauna Loa volcano hoping to divert a deadly lava flow away from Hilo | World News

On December 27, 1935, something unusual happened in the skies above Hawaiʻi. US military aircraft dropped 40 bombs, not on an enemy, but on a river of molten lava pouring down the slopes of Mauna Loa. The eruption had begun a month earlier inside the volcano’s summit caldera, and by late December, a fast-moving lava flow was heading straight for Hilo and the water supply that fed it. Officials were desperate. Scientists believed that explosives could break apart the underground channels carrying lava toward the town, forcing it to spread out and slow down. What followed became one of the earliest recorded attempts at lava flow diversion, and a story still argued over nearly a century later.

The Eruption That Threatened Hilo’s Water Supply

The Mauna Loa eruption started on November 21, 1935, inside the volcano’s summit caldera. Within days, activity shifted down the Northeast Rift Zone, and a new vent opened on the volcano’s northern flank on November 27. From this vent, smooth pāhoehoe lava began collecting in the saddle area between Mauna Loa and Mauna Kea before turning east toward Hilo. According to the US Geological Survey, the flow was advancing at roughly one mile a day, which put the Wailuku River, and the town’s water source, directly in its path.Thomas A. Jaggar, who had founded the Hawaiian Volcano Observatory, and fellow volcanologist Ruy Finch came up with an unconventional idea. They proposed using aerial bombing to rupture the lava channels near the vent itself. The goal was never to stop the eruption or destroy the lava outright. It was to break the insulated tubes and channels that let molten rock travel long distances without cooling, so the lava would spread thin across a wider area instead of racing toward Hilo in one concentrated stream.

Pāhoehoe Lava And The Science Behind The Plan

Pāhoehoe lava has a smooth, rope-like surface that cools quickly on the outside. But underneath that hardened crust, liquid lava can keep flowing through tubes and channels, insulated from the air and barely losing heat. This is exactly what allows a flow to travel for miles without solidifying. Jaggar and Finch reasoned that if bombs could tear open these channels, the lava would lose its protective covering, cool faster, and stall before reaching the river.It was not a brand new idea. Explosives had actually been proposed to stop lava flows threatening Hilo back in 1880 and 1881, though neither plan was carried out. The National Park Service describes the 1935 mission as the first time an active lava flow was ever bombed from the air anywhere in the world.

George Patton And The 40 Bombs Dropped On Mauna Loa

Overseeing the operation was Lieutenant Colonel George S. Patton, who would go on to lead US forces during the Second World War. Aircraft flew out from Oʻahu to the Humuʻula flow and released their payload near the lava channels on Mauna Loa’s northern flank. The mission used 20 demolition bombs and 20 smaller marker bombs meant to help pilots find their targets. Each demolition bomb weighed 600 pounds and packed 355 pounds of TNT, fitted with a short delay fuse so it could punch through the surface before exploding underneath.Of the demolition bombs, 16 reached the intended area, and 12 struck the lava tunnel they were aiming for. Jaggar watched the entire operation through a telescope positioned near Mauna Kea. He later called it a clear success, saying the results matched exactly what he had expected. The lava flow slowed down after the bombing and stopped completely by January 2, 1936.

Why Scientists Still Disagree About What Actually Worked

Not everyone was convinced. Howard Stearns, a USGS geologist who had flown along on the final bombing run, wrote in his 1983 autobiography that he believed the whole thing was simply a coincidence. The lava had kept moving even after the explosions, gradually losing strength over several days rather than stopping abruptly because of the bombs.This is really the heart of the debate. The eruption was already fading naturally around the same time the bombing took place, which makes it almost impossible to separate what the explosives actually did from what would have happened anyway. The USGS itself describes the effectiveness of using bombs to break lava channels as unresolved. A second bombing attempt in 1942 produced similarly unclear results.What the 1935 mission does represent, regardless of how one reads the outcome, is one of the earliest organised efforts by humans to influence where lava travels rather than trying to stop a volcano altogether. Nearly ninety years on, with modern instruments and far better understanding of how lava tubes behave, scientists are still cautious about claiming victory over something as unpredictable as an active volcano. Mauna Loa, it turns out, keeps its own counsel.