The lethal payloads rained down from Zeppelins — enormous airships, some the length of two football fields. With a team of engineers, explosives experts and historians, NOVA investigates the secrets behind these deadly war machines. The program explores the technological arms race that unfolded as Britain scrambled to develop defenses that could neutralize the threat, while Germany responded with ever bigger and more powerful Zeppelins. Why were these monsters of the sky, filled with flammable hydrogen gas, so difficult to shoot down? Experts reconstruct and detonate deadly WWI incendiary bombs and test-fire antique flaming bullets to discover how the British devised unique artillery that would finally take down the biggest flying machines ever made.