Saltpetre (saltpeter), or potassium nitrate, is a white crystal that easily dissolves in water. It has a wide range of industrial uses, but it is most famous for being the primary component of gunpowder. Saltpetre is highly combustible, which means it releases large amounts of energy along with toxic gases. And this is what happened in London, resulting in massive destruction and the loss of life. Similar catastrophes happened across the world in the age of gunpowder, one reason why officials decided to store explosives away from dense urban centers.