They are not really sure of what Greek Fire was. There are a lot of guesses, but the secret was guarded very closely and died out. Even our guesses do not seem to be very accurate because from the stories told about Greek Fire the reactions are not identical. Greek Fire was believed to be water proof and possibly even started by water. It caught quickly and could not be put out which made it very deadly to ships. It was a liquid and not a projectile, but that is what we know about it.