Tinian, a small island in the Northern Marianas near the Mariana Trench, was the secret assembly and launch site for Little Boy and Fat Man, the atomic bombs dropped on Hiroshima and Nagasaki. The Dive Detectives head offshore to a wartime dumpsite where the seafloor is scattered with unexploded ordnance and discarded bomb components, effectively a submerged museum of Project Alberta. Some of the material recovered contains phosphorous, and the film shows why that matters: local divers who have hauled the coral-like chunks to the surface have seen it ignite in open air, burning at up to 800 degrees Celsius. Underwater footage tracks the team as they identify wreckage and handle the hazardous finds with special precautions, while historical context fills in Tinian's role in the Manhattan Project's endgame. The episode moves between dive scenes and historical explanation, treating the seabed itself as physical evidence of the final months of the Pacific war.