The earliest extant written formula for gunpowder is in a Song Dynasty science manual. I forgot what the name of the writer was, though I had it written somewhere... By the time of the attempted Mongol invasions of Japan, Japanese sources described the Mongols and their Korean conscripts using grenades (those "thunder bombers" that were in Shogun: Total War
). Archeological findings off the coast of Japan confirmed this.
Of course, early gunpowder weapons were crappy (useless in rainy weather, and for grenades, you're just as likely to blow yourself up than the enemy), but they had quite an effect on enemy morale.
The Mahabharata also describes what is reminiscent of a nuclear explosion. VERY wierd.