The population of Indonesia grew dramatically after her people discovered iron sources and started to use iron tools. One of the oldest known iron sources is Luwu in South Sulawesi. Based on historical and archaeological records, Java's 13th to 15th century Majapahit Empire imported iron of exceptional quality, known as Pamor Luwu, for forging kris daggers. Research done in 2016 and 2018 by Indonesia's National Research Center for Archaeology confirms previously reported evidence that the primary source of Pamor Luwu is smelted ore from the Matano Lake environs. Complementing the remnants of iron production at terrestrial sites, we found evidence for it in an underwater site named Pulau Ampat. This research is the first underwater archaeology research on iron production in Asia, and our discovery broadens the knowledge of iron production in Indonesia dating to the 1st millennium AD.