Dense, superheated lava does whatever it wants. It certainly cannot be drained away. Few barriers can stand up to its incandescent anger. If lava threatens a harbor by the sea, pumping billions of gallons of seawater at it may slow it down, as Iceland discovered at Heimaey Island in 1973.

Image A pointer bomb dropped on Mauna Loa in 1935, photographed in 1977 by Jack Lockwood, a Hawaiian Volcano Observatory geologist. Credit... Jack Lockwood/U.S. Geological Survey

And then there is explosive lava diversion. As far back as 1881, it was considered to stop a lava flow headed toward Hilo. It was never tried; the pile of gunpowder remained unused, and religious conviction was widely credited with stopping the burning river.

The incendiary concept nevertheless struck a chord. Lava often travels long distances in solid tubes or in channels of its own design. Some wondered, why not sever these dangerous volcanic arteries? Bombs delivered by land could work, but aerial bombing could be more accurate and speedier.

It remained nothing more than a concept until an eruption in 1935. That December, a pond of lava breached its levees and advanced on Hilo at a rate of a mile per day. Fearing it would reach the town and its watershed, Thomas Jaggar, the founder and first director of the U.S. Geological Survey’s Hawaiian Volcano Observatory, called on the Army Air Corps. On Dec. 27, 10 Keystone B-3 and B-4 biplane bombers struck the lava flow, targeting its tubes and channels.

Half these bombs were packed with 355 pounds of TNT. The other half were not explosive, and instead designed to emit smoke so the pilots could see where the bona fide bombs landed. Mr. Singson found one of those inert devices last month.