Mauna Loa's "secret" eruption began on April 26, 1942. With World War II underway, nighttime blackouts were imposed on Hawai‘i. American officials feared that if the eruption were publicized, the Japanese military could use the bright glow of lava at night to guide warplanes to Hawai‘i. The eruption began on the western rim of Mauna Loa's summit caldera but then migrated down the volcano's Northeast Rift Zone. By the time the eruption ended on May 9, lava had reached to within 7 miles of the upper Waiākea Uka area of Hilo.