Swords in the ancient Middle East were made of a substance called Damascus steel, which was noted for its distinctive wavy pattern and famed for producing light, strong, and flexible blades. No one knows how it was made.

In defending Constantinople against the Muslims, the Byzantine Empire used something called “Greek fire,” an incendiary substance that was flung at the enemy’s ships and that burned all the more fiercely when wet. But precisely what it was, and how it was made, have been forgotten.

(Thanks, Mike.)