Greek Fire was known to be used by the Byzantine Empire(Eastern Roman Empire which mostly included the conquered Greeks). They were used majorly in the naval battles as the fire could burn when still on water. Greek Fire included a formula that is still not revealed today because it was kept as a state secret and the formula was lost when the Byzantine Empire collapsed. The formula for Greek Fire must include the ability to not be put out by water at all. It is said that only sand could put it out because sand deprive oxygen which is the main source of fuel for fires. Since ancient time buildings, ships, and sieges only included wood as their main material, and wood was rather flammable.