Imagine yourself a new recruit of the Arab navy around the 8th century. The smell of the sea is a comforting feeling, but not comforting enough it seems to your fellow sailors. You hear them mutter and mumble stories about what they call "liquid fire." You're not sure what they mean, and you're nervous to make waves with them, so you just listen. To you, this is just an ordinary military action. You're going to be a war hero, one of many sailors helping to take the city of Constantinople where the last attack had failed.
Midnight comes, and the order goes to begin the attack. As your ship approaches, you can see the Byzantine navy approaching. To you, their ships are outfitted rather strangely. There are things which look like catapults on many of the ships, and on a few of the faster clippers, you note large siphons of a sort. What could these people be planning, you wonder. Suddenly, light grows from each of the strangely equipped boats. The crew grows more nervous, but have no choice but to press on towards them.
The boats let loose flaming projectiles, and fear strikes your heart as well. Could this be the liquid fire that your fellows were wary of? Many of the projectiles in the first volley miss your navy's boats, but unbelievably, the fire stays alight on the water! How could this be? Then, as you're distracted, one of the smaller boats has come up to your ship, letting loose a veritable torrent of this bizarre flame out of the siphon. Screaming, panicking, you jump ship as the flames creep up the side. What is this terrible weapon of the Greek Empire?
Greek Fire was a weapon used by the Byzantine Empire (also known as the "Empire of the Greeks") starting at earliest around 673AD, and was one of the best kept, and most effective, military secrets in history. So well kept, in fact, that even today historians and scientists have only a few leads as to what this substance may have actually been. All they know is that it was probably stored in flasks, and behaved similarly to an oil fire, spreading when water was poured upon it, and burning even setting atop the ocean. However, unlike an oil fire, some stories and tales state that Greek Fire could burn even underwater, an impressive feat. Quicklime is a strong possibility for a chief reagent, reacting violently with water. It was possibly coupled with petroleum and sulfur to create a sticky, goopy mess that only got worse when water was used to try to extinguish it. However, no one can really know for sure.
Wikipedia, Greek Fire