The Byzantine Empire developed a weapon that was able to burn and be ignited by water. Nobody knows the formula anymore.

During the time of the Byzantine Empire, an incendiary weapon now most commonly called “Greek fire” was in high use. It gained notoriety because of its ability to burn floating on water, and even more impressively, be ignited by water. This made it highly useful in naval battles. 

Because it was such a useful weapon, the formula for Greek fire was a very tightly guarded state secret. This helped the Byzantine Empire by not letting their enemies use the weapon against them, but it also means that the formula has been lost to time. 

To this day, only speculation about its composition exists. In addition to burning on water, some accounts say only substances like sand or vinegar could extinguish it. It has even been said that the discharge of Greek fire was accompanied by thunder and smoke. It had some downsides, such as having limited range, and only being plausible to use under very specific circumstances.  

(Source)