In late 1944, Allied bomber crews flying night missions over the Rhine Valley began reporting something deeply unsettling: glowing balls of light, sometimes red, sometimes orange, sometimes a ghostly silver, that would pace their aircraft for minutes at a time, dart away at impossible speeds, and occasionally appear to formate in groups of eight or ten. The pilots of the 415th Night Fighter Squadron called them "foo fighters" — borrowing the nonsense word "foo" from the comic strip Smokey Stover, whose firefighter protagonist liked to say "where there's foo, there's fire."

What makes the foo fighter story irresistible isn't the sightings themselves — wartime skies were full of strange optical phenomena, from St. Elmo's fire to ball lightning to the afterimages of flak bursts. It's that both sides reported them. American crews assumed they were a secret Nazi weapon. German and Japanese pilots assumed they were a secret Allied weapon. After the war, intelligence officers searched captured documents expecting to find the program. They found nothing.

This is where the klystron sneaks in. One of the more elaborate post-war theories proposed that foo fighters were a German experimental anti-aircraft device, sometimes called the Feuerball ("fireball"). The supposed design: a disk-shaped drone propelled by gas jets arranged like a Catherine wheel, with miniature klystron tubes mounted inside to disrupt the ignition systems of Allied bombers via microwave interference. The glow, in this telling, was either ionized exhaust or the radiating klystron itself.

It's a wonderful piece of pulp engineering — and almost certainly fiction. The story traces back to a single Italian journalist, Renato Vesco, writing in the 1960s, with no documentary evidence behind it. But the choice of klystron as the magic ingredient is telling. By the 1960s, klystrons had become the iconic Cold War microwave tube: they powered radar arrays, they drove particle accelerators at SLAC, and they were the beating heart of early satellite uplinks. If you wanted a piece of mid-century technology that sounded both plausible and slightly sinister, "klystron" was the word you reached for. (The klystron was itself only invented in 1937, by the Varian brothers at Stanford — so the timeline for a 1944 German weaponization is tight but not impossible, which is exactly the kind of gap conspiracy theories thrive in.)

The likeliest mundane explanation for foo fighters is far more interesting than the Feuerball: a combination of St. Elmo's fire on bomber wingtips, ball lightning generated by engine ionization, reflections of ground flares off ice crystals, and — crucially — pilot fatigue and pattern-matching in crews who had been told by other crews to look for glowing orbs. Foo fighters effectively stopped appearing once the war ended and the squadrons disbanded.

The term didn't die, though. It became the direct linguistic ancestor of "UFO," shaped postwar flying-saucer mythology, and eventually gave Dave Grohl the name for his band.