The world's earliest bomb with an iron casing, the iron huopao 鐵火砲 (cast-iron bomb, also referred to as zhentian lei 震天雷, literally "thunder-crash bomb"), is one of the most important inventions of ancient China relating to gunpowder and firearms. Until recently, however, academics were not able to conduct substantial research on the topic, as no actual remains of a huopao had yet been found. Since 2013, in the excavation of two sites of Southern Song ancient cities (Diaoyu Fortress and Baidicheng) in Chongqing, archaeologists from the Chongqing Cultural Heritage Research Institute have discovered a number of iron huopao of the Southern Song dynasty. This finding not only provides first-hand material for research on the history of the invention of gunpowder and firearms, which has always been lacking, but also offers new physical evidence for an exploration of the history of war and the military history of the thirteenth century (in particular the Mongol-Song war), and is thus historically significant.