Greek fire was a chemical mixture which would burn on water. It was a dreaded weapon in the days of wooden ships -- for the sailors on the victim-ship could not douse the fire by pouring water on it.

Greek fire was particularly by the Byzantines. Its deadliness in combat, especially at sea, has been cited as a prime reason for the long survival of the Byzantine Empire in the face of many enemies. The art of compounding the mixture was a secret so closely guarded that its precise composition remains unknown to this day. [1997 Britannica as quoted on-line]

As Connie notes, it is believed that one ingredient was naptha.