HONOLULU (KHON2) — It’s the 80th anniversary of the eruption on Mauna Loa that lasted for 14 days and covered over 12 square miles of land.

On April 26, 1942, the summit crater Moku‘āweoweo on Mauna Loa began to erupt, a few months after the attack on Pearl Harbor. The National Park Service (NPS) said it became known as Mauna Loa’s “secret” eruption. Do you know the reason why?

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The 1942 event was noteworthy for several reasons, including that volcanologists were able to predict the timing and place of the eruption. This was also the second time lava diversion was tried on an eruption of Mauna Loa. However, what’s interesting about this eruption is that it was declared a secret.

The press was not allowed to publicize the event in fear that the Japanese military could use the bright glow of lava at night to guide warplanes to Hawaii. The eruption happened just a few months after the attack on Pearl Harbor, so with World War II underway, nighttime blackouts were in effect for the islands.

Aerial photograph of the 1942 Mauna Loa lava flow spreading downslope toward Hilo; smoke from burning trees in center of flow. Scattered cinder cones at the summit of Mauna Kea, top of photograph.
(Courtesy: USGS)

NPS cites Park Superintendent Wingate as saying, “Almost everyone expected the Japanese to follow up on their Pearl Harbor successes by an invasion attempt which could easily have been effective.”

In addition to the war-time restrictions, the lava flows threatened to engulf the water flume from Mauna Loa to the Army’s medical center and destroy ammunition depots in Hilo. By the time the eruption ended on May 9, lava had reached within seven miles of the upper Waiākea Uka area, NPS reported.

The military attempted to conduct bombing missions to select sites to change the course of the lava flows, but NPS said the bombs had little to no impact. A total of 16 bombs were dropped.

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Click here to see a table summarizing Mauna Loa activity over the past 200 years.