Trusdell recalls that within 24 hours of the last Mauna Loa eruption in 1984, plane tickets to the Island of Hawaii were sold out. “Everybody wanted to come to see the eruption,” he says.

An aerial view shows a bomb detonating on Mauna Loa during a 1935 eruption. The U.S. Army dropped 20 600-pound bombs on the lava flow that December morning to divert the molten rock from reaching ...