“Greek Fire” was an incendiary based weapon used by the Roman Empire, often during naval warfare.
It was a strong and intense flame, that had the unique ability to burn on water, making it a very powerful weapon in naval combat, as the ships would still be burning when struck by it, instead of being extinguished by the sea water.
The recipe for Greek Fire remains unknown, as it was a heavily guarded state secret. No one was told the recipe or had written it down, in case it was discovered by an enemy and used against them. However historians believe that the Greek Fire was made by combining crude oil with resins, making it a primitive version of Napalm