But regardless of whether Islamic State’s floating condom-bomb works, the concept is entirely sound. For more than 50 years, the United States, Russia and others have experimented with similar weapons.

Perhaps the most well known analogue is the barrage balloon. A simple device, the balloons are anchored to the ground with a heavy metal cable.

Strong enough to withstand the impact of an airplane, the line presents a dangerous hazard for enemy aircraft. A network of the unmanned blimps could protect cities or important military facilities.