Abstract In view of the “4·21” Indonesian submarine “Nanggala” crash event, an analysis of the marine environmental factors that may lead to the crash is carried out. The results show that the ocean internal wave is one possible cause of the disappearance of Indonesian submarines. The event is most likely that the submarine was subject to the so called “sea cliff” when encountering large-amplitude ocean internal waves, and at the same time, the officers and soldiers on board failed to take reasonable evasive measures in time, resulting in the submarine losing control and falling below the maximum safe diving depth, making the submarine crack under the pressure of the huge sea, resulting in a submarine accident. This conclusion is obtained based on the analysis of integrated ocean internal wave satellite images and numerical simulations.