'Ancient-looking' Bombs in Hawaii Volcano Rediscovered 85 Years after They Were Dropped to save Town from Lava

Bombs dropped into a volcano by the U.S. Army 85 years ago in a bid to divert lava from a nearby town have been rediscovered.

The bombs, which were dropped as part of a cluster in 1935, were reported by local media at the end of February, and Hawaii Volcano Observatory (HVO) has now explained the presence of these "ancient-looking bombs" for its weekly Volcano Watch article.

Last month, Hawaii News reported that Kawika Singson had found the unexploded bombs in a lava tube while he was exploring the lava fields of Mauna Loa, the largest volcano on Earth. Singson got in touch with the Department of Land and Natural Resources, which later confirmed the bomb was in a remote part of the reserve.

In a Facebook post, he shared images of the bombs, saying he "stumbled across" the bombs while hiking. He said he believed they were dropped in either 1935 or 1942 in one of the attempts made to divert lava from flowing into the town of Hilo, on the eastern side of the island of Hawaii.

HVO has now said the bombs Singson found were dropped on December 27, 1935. Twenty of the bombs dropped were demolition bombs loaded with 355 pounds of TNT. Another 20 were pointer bombs used to aim. "The bombs were part of an effort to divert lava. A month after the 1935 eruption of Mauna Loa began, the lava pond at the base of Mauna Kea breached, sending a flow toward Hilo," the HVO said in a Facebook post, adding the lava was advancing at over one mile per day.

Thomas A Jagger, the founder of HVO, had previously considered the idea of dropping bombs onto lava to divert its flow. He asked for assistance from the army, which granted the request. The bombs were dropped just over a mile from the vent of Mouna Loa.

volcano bomb
Image of the bomb in Mauna Loa in 1977. The Army dropped the bombs in an attempt to stop lava from threatening a nearby town. USGS photo by J. Lockwood

"Events leading up to bombs penetrating the thin pāhoehoe crust of the active 1935 Humu'ula lava flow are well known to volcanologists as the first test of using explosives to stall or divert a lava flow in Hawaii," the HVO article said. "Destroying a lava conduit to redirect a flow is one of three basic diversion tactics. However, lava diversion is the subject of great debate in Hawaii, with concerns about the success of influencing a lava flow's progress and whether humans should interfere with natural processes and Pele."

Humans first floated the idea of using bombs to prevent volcanoes from damaging nearby towns and cities in 1881, according to the HVO. This was also to prevent a Mauna Loa eruption from threatening Hilo. When the volcano started erupting in 1933, the idea resurfaced.

"A month after the 1935 eruption began, the lava pond at the base of Mauna Kea breached, sending a flow toward Hilo... For the previous two years, Jaggar had talked about using explosives to disrupt a lava flow that might threaten Hilo. He envisioned a land expedition carrying TNT to near the vent. But Jaggar's friend, sugar chemist Guido Giacometti, suggested that Army planes might be able to drop explosive bombs more quickly and accurately."

According to the U.S. Geological Survey (USGS), subsequent assessments of the bombing attempt were mixed. USGS geologist Harold Stearns said he did not believe the bombs had been effective. Hawaii National Park Superintendent E.G. Wingate also raised doubts and wrote to Army commanders to say he was looking at possible areas for a land expedition that would be more successful at changing the flow channel.

Jagger, however, believed the effort worked, saying the rate of the lava flow had been slowed from one mile per day to around 1,000 feet per day. "I have no question that this robbing of the source tunnel slowed down the movement of the front," he wrote at the time.

The eruption ended six days after the bombs were dropped and Hilo was not destroyed by the lava. Whether the bombs helped avert disaster is still disputed today, according to the USGS.