Between 1942 and 1944, the British Admiralty launched approximately 99,142 free-flying balloons from sites along the English coast. Operation Outward was brilliantly simple: cheap latex balloons filled with hydrogen carried either a trailing steel wire (designed to short-circuit power lines) or small incendiary devices (to start fires).
The balloons cost just 35 shillings each (about Ā£75 today) and were released when winds were favorable for reaching Germany. The wire-carrying balloons proved remarkably effective: they caused numerous power outages across Germany, including one that short-circuited a 110,000-volt power line near Leipzig, causing a fire that destroyed the entire power station.
The Germans expended considerable resources trying to counter the balloons, including diverting fighter aircraft and anti-aircraft batteries. A cost analysis showed that each balloon cost the British about 35 shillings but caused an estimated Ā£1.5 million in total damage across all operations. The program was one of the most cost-effective weapons of the war, though it has been largely forgotten.